ol/functions.js

OpenLayers mapping library

/**
 * @module ol/functions
 */
import { equals as arrayEquals } from './array.js';
/**
 * Always returns true.
 * @returns {boolean} true.
 */
export function TRUE() {
    return true;
}
/**
 * Always returns false.
 * @returns {boolean} false.
 */
export function FALSE() {
    return false;
}
/**
 * A reusable function, used e.g. as a default for callbacks.
 *
 * @return {void} Nothing.
 */
export function VOID() { }
/**
 * Wrap a function in another function that remembers the last return.  If the
 * returned function is called twice in a row with the same arguments and the same
 * this object, it will return the value from the first call in the second call.
 *
 * @param {function(...any): ReturnType} fn The function to memoize.
 * @return {function(...any): ReturnType} The memoized function.
 * @template ReturnType
 */
export function memoizeOne(fn) {
    var called = false;
    /** @type {ReturnType} */
    var lastResult;
    /** @type {Array<any>} */
    var lastArgs;
    var lastThis;
    return function () {
        var nextArgs = Array.prototype.slice.call(arguments);
        if (!called || this !== lastThis || !arrayEquals(nextArgs, lastArgs)) {
            called = true;
            lastThis = this;
            lastArgs = nextArgs;
            lastResult = fn.apply(this, arguments);
        }
        return lastResult;
    };
}
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