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<!DOCTYPE html>  <html lang="en"> <head> <meta charset="utf-8"> <title>CanJS - KeyLookup Expression</title> <meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1, user-scalable=no"> <link rel="stylesheet" type="text/css" href="../../static/bundles/bit-docs-site/static.css"> <link rel="shortcut icon" sizes="16x16 24x24 32x32 48x48 64x64" href="/docs/images/canjs_favicon.ico"> <link rel="apple-touch-icon" sizes="57x57" href="../../../docs/images/canjs_favicon_57x57.png"> <link rel="apple-touch-icon-precomposed" sizes="57x57" href="../../../docs/images/canjs_favicon_57x57.png"> <link rel="apple-touch-icon" sizes="72x72" href="../../../docs/images/canjs_favicon_72x72.png"> <link rel="apple-touch-icon" sizes="114x114" href="../../../docs/images/canjs_favicon_114x114.png"> <link rel="apple-touch-icon" sizes="120x120" href="../../../docs/images/canjs_favicon_128x128.png"> <link rel="apple-touch-icon" sizes="144x144" href="../../../docs/images/canjs_favicon_144x144.png"> <link rel="apple-touch-icon" sizes="152x152" href="../../../docs/images/canjs_favicon_152x152.png"> <meta content="yes" name="apple-mobile-web-app-capable"> <meta name="apple-mobile-web-app-status-bar-style" content="white-translucent"> <script> (function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){ (i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o), m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m) })(window,document,'script','https://www.google-analytics.com/analytics.js','ga'); ga('create', 'UA-2302003-11', 'auto'); ga('send', 'pageview'); </script> </head> <body> <input type="checkbox" id="nav-trigger" class="nav-trigger"/> <label for="nav-trigger">Menu</label> <div id="everything"> <div id="left" class="column"> <div class="top-left"> <div class="brand"> <div class="logo"> <a href="../../../index.html" alt="CanJS"></a> <div class="dropdown project-dropdown"> <a href="https://donejs.com/">DoneJS</a> <a href="http://stealjs.com/">StealJS</a> <a href="http://jquerypp.com/">jQuery ++</a> <a href="https://funcunit.com/">FuncUnit</a> <a href="http://documentjs.com/">DocumentJS</a> </div> </div> <div class="version"> <div class="version-number"> 3.0.0 </div> <div class="dropdown version-dropdown"> <a href="https://v2.canjs.com">2.3.27</a> </div> </div> </div> <div class="search-bar"> <p>   </p> </div> </div> <div class="bottom-left"> <div class="social-side-container"> <ul class="social-side"> <li> <a class="header-mobile github" href="https://github.com/canjs/canjs" target="_blank"><img class="social-icon-small" src="../../../docs/images/github.png">Github</a> </li> <li> <a class="header-mobile twitter" href="https://twitter.com/canjs" target="_blank"><img class="social-icon-small" src="../../../docs/images/twitter.png">Twitter</a> </li> </ul> <ul class="social-side"> <li> <a class="header-mobile" href="https://gitter.im/canjs/canjs" target="_blank">Chat</a> </li> <li> <a class="header-mobile" href="http://forums.donejs.com/c/canjs" target="_blank">Forum</a> </li> </ul> </div> <ul> <li class=" "> <a class="page" href="../../guides.html" title="Welcome to CanJS! 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These can be used in various combinations to call helper methods or viewModel methods. The following is an example of all the expressions combined:"> Expressions </a> </li> <li class=" "> <a class="page" href="../../can-stache.Acquisition.html" title="There are number of ways to acquire templates such as: raw text, a module loader, or script tags in the markup."> Template Acquisition </a> </li> <li class=" "> <a class="page" href="../../can-stache.Helpers.html" title="Helpers are functions that can be registered and called from within templates. 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Register a partial template that can be rendered with can-stache.tags.partial."> registerPartial </a> </li> <li class=" "> <a class="function" href="../../can-stache.registerSimpleHelper.html" title="Register a helper that gets passed values."> registerSimpleHelper </a> </li> <li class=" "> <a class="function" href="../../can-stache.safeString.html" title="Indicate that a string does not need to be escaped to be safely inserted into the page."> safeString </a> </li> </ul> </li> <li> <span>Tags</span> <ul> <li class=" "> <a class="function" href="../../can-stache.tags.escaped.html" title="Insert the value of the expression into the output of the template."> {{expression}} </a> </li> <li class=" "> <a class="function" href="../../can-stache.tags.unescaped.html" title="Insert the unescaped value of the expression into the output of the template."> {{{expression}}} </a> </li> <li class=" "> <a class="function" href="../../can-stache.tags.section.html" title="Renders a subsection one or more times depending on the type of expression or the expression's return value."> {{#expression}} </a> </li> <li class=" "> <a class="function" href="../../can-stache.tags.close.html" title=""> {{/expression}} </a> </li> <li class=" "> <a class="function" href="../../can-stache.tags.inverse.html" title="Like can-stache.tags.section, but renders the opposite subsection depending on the type of expression or the expression's return value."> {{^expression}} </a> </li> <li class=" "> <a class="function" href="../../can-stache.tags.partial.html" title="Render another template within the current template."> {{>key}} </a> </li> <li class=" "> <a class="function" href="../../can-stache.tags.comment.html" title="A comment that doesn't get inserted into the rendered result."> {{!expression}} </a> </li> <li class=" "> <a class="function" href="../../can-stache.helpers.else.html" title=""> {{else}} </a> </li> </ul> </li> <li> <span>Expressions</span> <ul> <li class=" "> <a class="typedef" href="bracket.html" title=""> Bracket Expression </a> </li> <li class=" "> <a class="typedef" href="call.html" title=""> Call Expression </a> </li> <li class=" "> <a class="typedef" href="hash.html" title=""> Hash Expression </a> </li> <li class=" "> <a class="typedef" href="helper.html" title=""> Helper Expression </a> </li> <li class="current parent expanded"> <a class="typedef" href="key-lookup.html" title=""> KeyLookup Expression </a> </li> <li class=" "> <a class="typedef" href="literal.html" title="Specify a JavaScript primitive type."> Literal Expression </a> </li> </ul> </li> <li> <span>Key Operators</span> <ul> <li class=" "> <a class="typedef" href="../keys/at.html" title="Return whatever value is at a key, regardless if it's a function or a compute."> @at </a> </li> <li class=" "> <a class="typedef" href="../keys/compute.html" title="Pass a compute instead of a value if an observable is found within calls."> ~compute </a> </li> <li class=" "> <a class="typedef" href="../keys/current.html" title="Lookup a value in only the current context."> ./current </a> </li> <li class=" "> <a class="typedef" href="../keys/parent.html" title="Start looking for values in the parent context."> ../parent </a> </li> <li class=" "> <a class="typedef" href="../keys/special.html" title="Event bindings and some helpers like can-stache.helpers.each provide special values that start with % to prevent potential collisions with other values."> %special </a> </li> <li class=" "> <a class="typedef" href="../keys/this.html" title="Write out or return the current context."> this </a> </li> <li class=" "> <a class="typedef" href="../keys/variable.html" title="Store a variable local to the template."> *variable </a> </li> <li class=" "> <a class="typedef" href="../../can-stache.key.html" title="A named reference to a value in the scope or helper scope in a template."> key </a> </li> </ul> </li> <li> <span>Helpers</span> <ul> <li class=" "> <a class="function" href="../../can-stache.helpers.if.html" title="can-stache.helpers.if"> {{#if expression}} </a> </li> <li class=" "> <a class="function" href="../../can-stache.helpers.unless.html" title=""> {{#unless expression}} </a> </li> <li class=" "> <a class="function" href="../../can-stache.helpers.each.html" title=""> {{#each expression}} </a> </li> <li class=" "> <a class="function" href="../../can-stache.helpers.with.html" title="Changes the context within a block."> {{#with expression}} </a> </li> <li class=" "> <a class="function" href="../../can-stache.helpers.log.html" title=""> {{log}} </a> </li> <li class=" "> <a class="function" href="../../can-stache.helpers.is.html" title="Render something if two values are equal."> {{#is expressions}} </a> </li> <li class=" "> <a class="function" href="../../can-stache.helpers.switch.html" title=""> {{#switch expression}} </a> </li> <li class=" "> <a class="function" href="../../can-stache.helpers.case.html" title=""> {{#case expression}} </a> </li> <li class=" "> <a class="function" href="../../can-stache.helpers.default.html" title=""> {{#default}} </a> </li> <li class=" "> <a class="function" href="../../can-stache.helpers.joinBase.html" title=""> {{joinBase expressions}} </a> </li> </ul> </li> <li> <span>Types</span> <ul> <li class=" "> <a class="typedef" href="../../can-stache.getterSetter.html" title="The getterSetter argument passed to registerConverter."> getterSetter </a> </li> <li class=" "> <a class="typedef" href="../../can-stache.helper.html" title="A helper function passed to can-stache.registerHelper. Given the arguments, returns the content that should be shown in the DOM or a function that will be called on the DOM element the helper was called on."> helper </a> </li> <li class=" "> <a class="typedef" href="../../can-stache.helperOptions.html" title="The options argument passed to a helper function when called by a helper."> helperOptions </a> </li> <li class=" "> <a class="typedef" href="../../can-stache.renderer.html" title="A function returned by can-stache that renders a template into an html documentFragment."> renderer </a> </li> <li class=" "> <a class="typedef" href="../../can-stache.sectionRenderer.html" title="Renders a section. These functions are usually provided as .fn and .inverse on a stache helper's options."> sectionRenderer </a> </li> <li class=" "> <a class="typedef" href="../../can-stache.simpleHelper.html" title="A helper function passed to can-stache.registerSimpleHelper."> simpleHelper </a> </li> </ul> </li> </ul> </li> <li class=" "> <a class="module" href="../helpers/route.html" title="Adds stache helpers that use can-route."> can-stache/helpers/route </a> </li> <li class=" "> <a class="module" href="../../can-stache-bindings.html" title="Provides template event, one-way, and two-way bindings."> can-stache-bindings </a> </li> </ul> </li> <li class=" "> <a class="page" href="../../can-ecosystem.html" title="Useful libraries that extend or add important features to the core collection."> Ecosystem </a> </li> <li class=" "> <a class="page" href="../../can-infrastructure.html" title="Utility libraries that power the core and ecosystem collection."> Infrastructure </a> </li> <li class=" "> <a class="page" href="../../can-legacy.html" title="Former libraries that we still accept patches for, but are not under active development."> Legacy </a> </li> </ul> </div> </div> <div id="right" class="column"> <div class="top-right"> <div class="top-right-top"> <ul class="top-right-bitovi"> <li class="dropdown"> <a href="http://bitovi.com" class="bitovi icon-bits">Bitovi</a> <ul class="dropdown-menu"> <li><a href="http://bitovi.com">Bitovi.com</a></li> <li><a href="http://bitovi.com/blog/">Blog</a></li> <li><a href="http://bitovi.com/consulting/">Consulting</a></li> <li><a href="http://bitovi.com/training/">Training</a></li> <li><a href="http://bitovi.com/open-source/">Open Source</a></li> </ul> </li> </ul> <div class="brand"> <div class="logo"> <a href="../../../index.html" alt="CanJS"></a> </div> </div> <ul class="top-right-links"> <li> <a href="https://gitter.im/canjs/canjs">Chat</a> </li> <li> <a href="http://forums.donejs.com/c/canjs">Forum</a> </li> <li> <a class="github-button nav-social" href="https://github.com/canjs/canjs" data-count-href="/canjs/canjs/stargazers" data-count-api="/repos/canjs/canjs#stargazers_count">Star</a> </li> <li> <a href="https://twitter.com/canjs" class="twitter-follow-button nav-social" data-show-count="true" data-show-screen-name="false">Follow @canjs</a><script async src="//platform.twitter.com/widgets.js" charset="utf-8"></script> </li> </ul> </div> <div class="breadcrumb"> <li><a href="../../../index.html">CanJS</a></li> / <li><a href="../../can-core.html">Core</a></li> / <li><a href="../../can-stache.html">can-stache</a></li> / <li><a href="key-lookup.html">KeyLookup Expression</a></li> <li class="breadcrumb-dropdown">/ <a> On this page</a> <ul class="on-this-page"></ul> </li> <div class="nav-toggle" title="Back to top"></div> </div> </div> <div class="bottom-right"> <article> <section class="title"> <div class="page-type"> <h1>KeyLookup Expression</h1> <div>typedef</div> </div> <section class="description"> </section> </section> <section class="on-this-page-table"> </section> <section class="title-footer"> <ul class="title-links">  <li><a href="//github.com/canjs/can-stache/tree/v3.0.8/docs/expressions/key-lookup.md">source</a></li>   </ul> </section> <div class="signature"> <h2 class="signature-title"> <code>key</code> </h2> <p>A <a href="../../can-stache.key.html" title="A named reference to a value in the scope or helper scope in a template.">KeyLookup expression</a> specifies a value in the <a href="../../can-view-scope.html" title="Create a lookup node for keys.">scope</a> or <a href="../../can-view-scope.Options.html" title="">HelperOptions scope</a> that will be looked up. KeyLookup expressions can be the entire stache expression like:</p> <pre><code>{{key}} </code></pre> <p>Or they can makeup the method, helper, arguments and hash value parts of <a href="call.html" title="">Call Expression</a>, <a href="helper.html" title="">Helper Expression</a>, and <a href="hash.html" title="">Hash Expression</a>s:</p> <pre><code>{{method(arg1,arg2}} Call {{helper arg1 arg2}} Helper {{method( prop=hashValue )}} Hash </code></pre> </div> <div class="signature"> <h2 class="signature-title"> <code>CALL_EXPRESSION.key</code> </h2> <p>Looks up <code>key</code> on the return value of <code>CALL_EXPRESSION</code>.</p> <pre><code>{{#each Todo.getList(complete=completed).value}} </code></pre> <div class="parameters"> <h3 class="parameters-title">Parameters</h3> <ol> <li><b>CALL_EXPRESSION</b> <code>{<a href="call.html" title="">Call Expression</a>}</code>: <p>A call expression that may or may not return a value.</p> </li> <li><b>key</b> <code>{String}</code>: <p>A property value to look up on the result of <code>CALL_EXPRESSION</code>.</p> </li> </ol> </div> </div> <section class="body"> <h2>Use</h2> <p>The value returned up by a KeyLookup depends on what the <a href="../../can-stache.key.html" title="A named reference to a value in the scope or helper scope in a template.">key</a> looks like, and what expression type the KeyLookup is within.</p> <p>For example, <code>{{method(~./key)}}</code> will call <code>method</code> with a <a href="../../can-compute.computed.html" title="A derived value from other computes and observable maps.">compute</a> that looks up the value of <code>key</code> only in the top of the <a href="../../can-view-scope.html" title="Create a lookup node for keys.">scope</a>.</p> <p>The rules are as follows:</p> <ul> <li><strong><a href="call.html" title="">Call Expression</a> arguments</strong> <code>{{method(key)}}</code> - values are passed.</li> <li><strong><a href="helper.html" title="">Helper Expression</a> arguments</strong> <code>{{helper key}}</code> - computes are passed.</li> <li><strong>hash value in <a href="call.html" title="">Call Expression</a></strong> <code>{{method(hash=key)}}</code> - values are set as property values.</li> <li><strong>hash value in <a href="helper.html" title="">Helper Expression</a></strong> <code>{{method hash=key}}</code> - computes are set as property values.</li> <li><strong><a href="../keys/special.html" title="Event bindings and some helpers like can-stache.helpers.each provide special values that start with % to prevent potential collisions with other values.">special operator</a></strong> <code>{{%index}}</code> - lookup values in a special context provided by some helpers.</li> <li><strong><a href="../keys/compute.html" title="Pass a compute instead of a value if an observable is found within calls.">compute operator</a></strong> <code>{{method(~key)}}</code> - pass a compute instead of a value.</li> <li><strong><a href="../keys/special.html" title="Event bindings and some helpers like can-stache.helpers.each provide special values that start with % to prevent potential collisions with other values.">at operator</a></strong> <code>{{method(@key}}</code> - pass a function instead of trying to read the value of the function.</li> <li><strong><a href="../keys/current.html" title="Lookup a value in only the current context.">current operator</a></strong> <code>{{./key}}</code> - only lookup key at the top of the scope.</li> <li><strong><a href="../keys/parent.html" title="Start looking for values in the parent context.">parent operator</a></strong> <code>{{../key}}</code> - lookup the value in the parent context.</li> <li><strong><a href="../keys/this.html" title="Write out or return the current context.">context key</a></strong> <code>{{this}}</code> - return the current context/top of the scope.</li> </ul> <h2>Default key return values by expression and data types</h2> <p>Keys can have slightly different default behavior depending if they are used in:</p> <ul> <li><a href="helper.html" title="">helper arguments</a> like: <code>{{helper some.key}}</code></li> </ul> <p>when compared to the other places they are used:</p> <ul> <li><a href="../../can-stache.tags.escaped.html" title="Insert the value of the expression into the output of the template.">insertion tags</a> like: <code>{{some.key}}</code></li> <li><a href="call.html" title="">call-expression arguments</a> like: <code>{{helper(some.key)}}</code></li> <li><a href="../../can-stache-bindings.event.html" title="Response to events on elements or component ViewModels.">event bindings</a> like: <code>($click)="method(some.key)"</code></li> <li><a href="../../can-stache-bindings.html" title="Provides template event, one-way, and two-way bindings.">data bindings</a> like: <code>{some-attr}="some.key"</code></li> </ul> <p>Furthermore keys return different values depending on the data type.</p> <p>In general:</p> <ul> <li>Functions are called to get their return value. (Use the <a href="../keys/at.html" title="Return whatever value is at a key, regardless if it's a function or a compute."><code>@</code> operator</a> to prevent this).</li> <li>Keys in helper expression arguments that find observable data return a <a href="../../can-compute.computed.html" title="A derived value from other computes and observable maps.">compute</a> that represents the value.</li> <li>Keys in other expressions return the value.</li> <li>If no observable data is found, the key's value is returned in all expressions.</li> </ul> <p>The following illustrates what <code>some.key</code> would return given different data structures as a <a href="helper.html" title="">Helper Expression</a> and in all other expressions.</p> <pre><code>// A non-observable JS object: {some: {key: "value"}}; // Helper -> "value" // Other -> "value" // A non-observable JS object w/ a function at the end {some: {key: function(){ return "value"; }}} // Helper -> "value" // Other -> "value" // A non-observable JS object with intermeidate functions: {some: function(){ return {key: "value"}}} // Helper -> "value" // Other -> "value" // A observable can-map {some: new Map({key: "value"})} // Helper -> canCompute("value") // Other -> "value" // A method on an observable can-map that reads observables var Some = Map.extend({key: function(){ return this.attr("value")}}) {some: new Some({value: "value"})} // Helper -> canCompute("value") // Other -> "value" </code></pre> </section> <script type="text/javascript"> var docObject = {"src":{"path":"node_modules/can-stache/docs/expressions/key-lookup.md"},"description":"\n","type":"typedef","title":"KeyLookup Expression","types":[{"type":"String"}],"name":"can-stache/expressions/key-lookup","parent":"can-stache/expressions","signatures":[{"code":"key","description":"\n\nA [can-stache.key KeyLookup expression] specifies a value in the [can-view-scope scope] or\n[can-view-scope.Options HelperOptions scope] that will be looked up. KeyLookup expressions\ncan be the entire stache expression like:\n\n```\n{{key}}\n```\n\nOr they can makeup the method, helper, arguments and hash value parts of\n[can-stache/expressions/call],\n[can-stache/expressions/helper], and [can-stache/expressions/hash]s:\n\n```\n{{method(arg1,arg2}} Call\n{{helper arg1 arg2}} Helper\n{{method( prop=hashValue )}} Hash\n```\n","params":[]},{"code":"CALL_EXPRESSION.key","description":"\n\nLooks up `key` on the return value of `CALL_EXPRESSION`.\n\n```\n{{#each Todo.getList(complete=completed).value}}\n```\n","params":[{"types":[{"type":"can-stache/expressions/call"}],"name":"CALL_EXPRESSION","description":"A\ncall expression that may or may not return a value.\n"},{"types":[{"type":"String"}],"name":"key","description":"A property value to look up on\nthe result of `CALL_EXPRESSION`.\n"}]}],"_curParam":{"types":[{"type":"String"}],"name":"key","description":"A property value to look up on\nthe result of `CALL_EXPRESSION`.\n"},"comment":" ","pathToRoot":"../../.."}; </script> </article> <footer><p>CanJS is part of <a href="http://donejs.com" target="_blank">DoneJS</a>. 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