An ESnext spec-compliant Map.prototype.getOrInsertComputed shim/polyfill/replacement that works as far down as ES3.
This package implements the es-shim API v3 interface. It works in an ES3-supported environment and complies with the proposed spec.
npm install --save map.prototype.getorinsertcomputedvar getOrInsertComputed = require('map.prototype.getorinsertcomputed');
var assert = require('assert');
var map = new Map();
var key = {};
var value = {};
assert.equal(map.has(key), false);
assert.equal(getOrInsertComputed(map, key, () => value), value);
assert.equal(map.has(key), true);var getPolyfill = require('map.prototype.getorinsertcomputed/polyfill');
var shim = require('map.prototype.getorinsertcomputed/shim');
var assert = require('assert');
/* when Map.prototype.getOrInsertComputed is not present */
delete Map.prototype.getOrInsertComputed;
var shimmed = shim();
assert.equal(shimmed, getPolyfill());
var map = new Map();
var key = {};
var value = {};
assert.equal(map.has(key), false);
assert.equal(map.getOrInsertComputed(key, () => value), value);
assert.equal(map.has(key), true);var shim = require('map.prototype.getorinsertcomputed/shim');
var assert = require('assert');
/* when Map.prototype.getOrInsertComputed is present */
var shimmed = shim();
assert.equal(shimmed, Map.prototype.getOrInsertComputed);
var map = new Map();
var key = {};
var value = {};
assert.equal(map.has(key), false);
assert.equal(map.getOrInsertComputed(key, () => value), value);
assert.equal(map.has(key), true);Simply clone the repo, npm install, and run npm test
