index.js

(function () {
    'use strict';

    // modified from https://github.com/ljharb/object.assign
    if (Object.assign && Object.preventExtensions) {
        var assignHasPendingExceptions = (function () {
            // Firefox 37 still has "pending exception" logic in its Object.assign implementation,
            // which is 72% slower than our shim, and Firefox 40's native implementation.
            var thrower = Object.preventExtensions({1: 2});
            try {
                Object.assign(thrower, 'xy');
            } catch (e) {
                return thrower[1] === 'y';
            }
        }());
        if (assignHasPendingExceptions) {
            delete Object.assign;
        }
    }

    if (!Object.assign) {
        var keys = Object.keys;
        var defineProperty = Object.defineProperty;
        var canBeObject = function (obj) {
            return typeof obj !== 'undefined' && obj !== null;
        };
        var hasSymbols = typeof Symbol === 'function' && typeof Symbol() === 'symbol';
        var propIsEnumerable = Object.prototype.propertyIsEnumerable;
        var isEnumerableOn = function (obj) {
            return function isEnumerable(prop) {
                return propIsEnumerable.call(obj, prop);
            };
        };

        // per ES6 spec, this function has to have a length of 2
        var assignShim = function assign(target, source1) { //eslint-disable-line no-unused-vars
            if (!canBeObject(target)) {
                throw new TypeError('target must be an object');
            }
            var objTarget = Object(target);
            var s, source, i, props;
            for (s = 1; s < arguments.length; ++s) {
                source = Object(arguments[s]);
                props = keys(source);
                if (hasSymbols && Object.getOwnPropertySymbols) {
                    props.push.apply(props, Object.getOwnPropertySymbols(source).filter(isEnumerableOn(source)));
                }
                for (i = 0; i < props.length; ++i) {
                    objTarget[props[i]] = source[props[i]];
                }
            }
            return objTarget;
        };

        defineProperty(Object, 'assign', {
            value: assignShim,
            configurable: true,
            enumerable: false,
            writable: true
        });
    }
}());