__spread does not work in IE11

[nvirth]

This TypeScript code (a Jasmine test):

describe("array spread operator", function() {
    it('should be able to spread arguments', function() {
        let argumentsArray;
        function testFunction() {
            argumentsArray = [...arguments];
        }
        (testFunction as any)(1, 2, 3);
        expect(argumentsArray).toEqual([1, 2, 3]);
    });
});

runs successfully in browsers but IE.
In IE11, it fails with:

Expected $.length = 1 to equal 3.
Expected $[0] = [object Arguments] to equal 1.
Expected $[1] = undefined to equal 2.
Expected $[2] = undefined to equal 3.

I can see the ... array spread operator gets built into a call for helper __spread, thus submitting the issue here.

Browsers I tried:

• Chrome 76.0.3809.100 64bit
• Opera 62.0.3331.116
• Firefox Quantum 68.0.1 64-bit
• Internet Explorer 11.309.16299.0
• Microsoft Edge 41.16299.248.0

[RyanCavanaugh]

@rbuckton any idea what's up?

[imasalygin]

@RyanCavanaugh

IE doesn't support Symbol.Iterator
therefore function __read return arguments
therefore ar = [].concat(arguments) equals [arguments]
e.g., function function f() { return [...arguments] }; f(1); returns [{0: 1}]

[rbuckton]

We can possibly just change __read to do this:

    __read = function (o, n) {
        var m = typeof Symbol === "function" && o[Symbol.iterator];
        if (!m) return Array.prototype.slice.call(o, 0, n);
        ...

[rbuckton]

Previously we didn't care about the n argument here since its purpose was to limit how many times we step through the iterator in cases like [a, b] = [1, 2, 3]. It didn't matter when we were just passing through the input object, but if we end up copying it using slice then we should limit the size of the output to cut down on memory usage.

[anton-bot]

The solution is to polyfill the Symbol.

import 'core-js/features/symbol';

Source: https://www.leereamsnyder.com/blog/new-skool-uniq-in-internet-explorer

[nvirth]

We recently updated TSC from 4.1.2 to 4.4.4, where we could see diffing the built JS before and after the TSC upgrade, that lot of __spread usages have been replaced with __spreadArray.
After the upgrade, the above initial test case successfully runs in IE11.