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Fix/object is polyfill #2793

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Mar 2, 2021
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Fix/object is polyfill #2793

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c310c69
clarify Object.is() behavior as compared to === operator
chengjieyun59 Mar 2, 2021
e45edd4
explain Polyfill code and match case numbers to former section
chengjieyun59 Mar 2, 2021
a9366ee
Fix spellings from code review
chengjieyun59 Mar 2, 2021
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56 changes: 34 additions & 22 deletions files/en-us/web/javascript/reference/global_objects/object/is/index.html
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Expand Up @@ -68,32 +68,39 @@ <h2 id="Description">Description</h2>

<p>This is also <em>not</em> the same as being equal according to the
{{jsxref("Operators/Comparison_Operators", "===", "#Identity")}} operator. The
<code>===</code> operator (and the <code>==</code> operator as well) treats the number
values <code>-0</code> and <code>+0</code> as equal and treats {{jsxref("Number.NaN")}}
as not equal to {{jsxref("NaN")}}.</p>
only difference between <code>Object.is()</code> and <code>===</code> is in
their treatment of signed zeroes and NaNs. For example, the <code>===</code>
operator (and the <code>==</code> operator) treats the number values <code>-0</code>
and <code>+0</code> as equal. Also, the <code>===</code> operator treats
{{jsxref("Number.NaN")}} and {{jsxref("NaN")}} as not equal.</p>

<h2 id="Examples">Examples</h2>

<h3 id="Using_Object.is">Using Object.is</h3>

<pre class="brush: js">Object.is('foo', 'foo'); // true
Object.is(window, window); // true

Object.is('foo', 'bar'); // false
Object.is([], []); // false

<pre class="brush: js">// Case 1: Evaluation result is the same as using ===
Object.is(25, 25); // true
Object.is('foo', 'foo'); // true
Object.is('foo', 'bar'); // false
Object.is(null, null); // true
Object.is(undefined, undefined); // true
Object.is(window, window); // true
Object.is([], []); // false
var foo = { a: 1 };
var bar = { a: 1 };
Object.is(foo, foo); // true
Object.is(foo, bar); // false

Object.is(null, null); // true

// Special Cases
Object.is(0, -0); // false
Object.is(0n, -0n); // true
Object.is(-0, -0); // true
Object.is(NaN, 0/0); // true
Object.is(foo, foo); // true
Object.is(foo, bar); // false

// Case 2: Signed zero
Object.is(0, -0); // false
Object.is(+0, -0); // false
Object.is(-0, -0); // true
const n = 15;
Object.is(0n, -0n); // true

// Case 3: NaN
Object.is(NaN, 0/0); // true
Object.is(NaN, Number.NaN) // true
</pre>

<h2 id="Polyfill">Polyfill</h2>
Expand All @@ -102,11 +109,16 @@ <h2 id="Polyfill">Polyfill</h2>
  Object.defineProperty(Object, "is", {
  value: function (x, y) {
// SameValue algorithm
if (x === y) { // Steps 1-5, 7-10
// Steps 6.b-6.e: +0 != -0
if (x === y) {
// return true if x and y are not 0, OR
// if x and y are both 0 of the same sign.
// This checks for cases 1 and 2 above.
return x !== 0 || 1 / x === 1 / y;
} else {
// Step 6.a: NaN == NaN
// return true if both x AND y evaluate to NaN.
// The only possibility for a variable to not be strictly equal to itself
// is when that variable evaluates to NaN (example: Number.NaN, 0/0, NaN).
// This checks for case 3.
return x !== x &amp;&amp; y !== y;
}
  }
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