toPrimitive.js - mozsearch

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// ES6 20.3.4.45 Date.prototype[@@toPrimitive](hint)

// The toPrimitive method throws if the this value isn't an object.

var toPrimitive = Date.prototype[Symbol.toPrimitive];

assertThrowsInstanceOf(() => toPrimitive.call(undefined, "default"), TypeError);

assertThrowsInstanceOf(() => toPrimitive.call(3, "default"), TypeError);

// It doesn't have to be a Date object, though.

var obj = {

    toString() { return "str"; },

    valueOf() { return "val"; }

};

assertEq(toPrimitive.call(obj, "number"), "val");

assertEq(toPrimitive.call(obj, "string"), "str");

assertEq(toPrimitive.call(obj, "default"), "str");

// It throws if the hint argument is missing or not one of the three allowed values.

assertThrowsInstanceOf(() => toPrimitive.call(obj), TypeError);

assertThrowsInstanceOf(() => toPrimitive.call(obj, undefined), TypeError);

assertThrowsInstanceOf(() => toPrimitive.call(obj, "boolean"), TypeError);

assertThrowsInstanceOf(() => toPrimitive.call(obj, ["number"]), TypeError);

assertThrowsInstanceOf(() => toPrimitive.call(obj, {toString() { throw "FAIL"; }}), TypeError);

// The next few tests cover the OrdinaryToPrimitive algorithm, specified in

// ES6 7.1.1 ToPrimitive(input [, PreferredType]).

// Date.prototype.toString or .valueOf can be overridden.

var dateobj = new Date();

Date.prototype.toString = function () {

    assertEq(this, dateobj);

    return 14;

};

Date.prototype.valueOf = function () {

    return "92";

};

assertEq(dateobj[Symbol.toPrimitive]("number"), "92");

assertEq(dateobj[Symbol.toPrimitive]("string"), 14);

assertEq(dateobj[Symbol.toPrimitive]("default"), 14);

assertEq(dateobj == 14, true);  // equality comparison: passes "default"

// If this.toString is a non-callable value, this.valueOf is called instead.

Date.prototype.toString = {};

assertEq(dateobj[Symbol.toPrimitive]("string"), "92");

assertEq(dateobj[Symbol.toPrimitive]("default"), "92");

// And vice versa.

Date.prototype.toString = function () { return 15; };

Date.prototype.valueOf = "ponies";

assertEq(dateobj[Symbol.toPrimitive]("number"), 15);

// If neither is callable, it throws a TypeError.

Date.prototype.toString = "ponies";

assertThrowsInstanceOf(() => dateobj[Symbol.toPrimitive]("default"), TypeError);

// Surface features.

assertEq(toPrimitive.name, "[Symbol.toPrimitive]");

var desc = Object.getOwnPropertyDescriptor(Date.prototype, Symbol.toPrimitive);

assertEq(desc.configurable, true);

assertEq(desc.enumerable, false);

assertEq(desc.writable, false);

reportCompare(0, 0);