streaming/mapper.js

Transforms and other streaming helpers

"use strict";
var __extends = (this && this.__extends) || (function () {
    var extendStatics = function (d, b) {
        extendStatics = Object.setPrototypeOf ||
            ({ __proto__: [] } instanceof Array && function (d, b) { d.__proto__ = b; }) ||
            function (d, b) { for (var p in b) if (Object.prototype.hasOwnProperty.call(b, p)) d[p] = b[p]; };
        return extendStatics(d, b);
    };
    return function (d, b) {
        if (typeof b !== "function" && b !== null)
            throw new TypeError("Class extends value " + String(b) + " is not a constructor or null");
        extendStatics(d, b);
        function __() { this.constructor = d; }
        d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
    };
})();
Object.defineProperty(exports, "__esModule", { value: true });
exports.Mapper = void 0;
var stream_1 = require("stream");
var __null = {
    toJSON: function () {
        return null;
    },
    valueOf: function () {
        return null;
    },
};
/**
Mapper transforms a stream of <T> into a stream of <R>, using a synchronous
transform function.

Use Transformer() instead if you need an asynchronous callback function.
*/
var Mapper = /** @class */ (function (_super) {
    __extends(Mapper, _super);
    function Mapper(transformFn) {
        var _this = _super.call(this, { objectMode: true }) || this;
        _this.transformFn = transformFn;
        _this.transformFn = transformFn.bind(_this);
        return _this;
    }
    Mapper.prototype._transform = function (chunk, encoding, callback) {
        var result = this.transformFn(chunk);
        if (result === null) {
            // We have to wrap pure JS nulls, because `push(null)` means EOF to streams.
            // It's kind of a hack, but it works nicely when we JSON.stringify downstream.
            this.push(__null);
        }
        else if (result !== undefined) {
            this.push(result);
        }
        // otherwise skip it; undefined denotes no output
        callback();
    };
    return Mapper;
}(stream_1.Transform));
exports.Mapper = Mapper;