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<code>String</code>, called as a function, converts its argument to a <code>string</code>. <code>String</code>, called as a constructor, creates an object whose prototype is the <code>String</code> function. (Check <a href="https://stackoverflow.com/a/11977606/15055">James's Answer</a> for the relevant ECMAScript specification section.) This is indeed confusing. The two equality operators actually do very different things. From the <a href="http://www.ecma-international.org/ecma-262/5.1/Ecma-262.pdf" rel="nofollow noreferrer">ECMA-262, v 5.1 document</a>, <code>===</code> does: <blockquote> <ol> <li>If <code>Type(x)</code> is different from <code>Type(y)</code>, return <code>false</code>. </li> <li>If <code>Type(x)</code> is <code>Undefined</code>, return <code>true</code>. </li> <li>If <code>Type(x)</code> is <code>Null</code>, return <code>true</code>.</li> <li>If <code>Type(x)</code> is <code>Number</code>, then a. If <code>x</code> is <code>NaN</code>, return <code>false</code>. b. If <code>y</code> is <code>NaN</code>, return <code>false</code>. c. If <code>x</code> is the same <code>Number</code> value as <code>y</code>, return <code>true</code>. d. If <code>x</code> is <code>+0</code> and <code>y</code> is <code>-0</code>, return <code>true</code>. e. If <code>x</code> is <code>-0</code> and <code>y</code> is <code>+0</code>, return <code>true</code>. f. Return <code>false</code>.</li> <li>If <code>Type(x)</code> is <code>String</code>, then return <code>true</code> if <code>x</code> and <code>y</code> are exactly the same sequence of characters (same length and same characters in corresponding positions); otherwise, return <code>false</code>.</li> <li>If <code>Type(x)</code> is <code>Boolean</code>, return <code>true</code> if <code>x</code> and <code>y</code> are both <code>true</code> or both <code>false</code>; otherwise, return <code>false</code>.</li> <li>Return <code>true</code> if <code>x</code> and <code>y</code> refer to the same object. Otherwise, return <code>false</code>.</li> </ol> </blockquote> Whereas <code>==</code> does: <blockquote> <ol> <li>If <code>Type(x)</code> is the same as <code>Type(y)</code>, then a. If <code>Type(x)</code> is <code>Undefined</code>, return <code>true</code>. b. If <code>Type(x)</code> is <code>Null</code>, return <code>true</code>. c. If <code>Type(x)</code> is <code>Number</code>, then    i. If <code>x</code> is <code>NaN</code>, return <code>false</code>.    ii. If <code>y</code> is <code>NaN</code>, return <code>false</code>.    iii. If <code>x</code> is the same <code>Number</code> value as <code>y</code>, return <code>true</code>.    iv. If <code>x</code> is <code>+0</code> and <code>y</code> is <code>-0</code>, return <code>true</code>.    v. If <code>x</code> is <code>-0</code> and <code>y</code> is <code>+0</code>, return <code>true</code>.    vi. Return <code>false</code>. d. If <code>Type(x)</code> is <code>String</code>, then return <code>true</code> if <code>x</code> and <code>y</code> are exactly the same sequence of characters (same length and same characters in corresponding positions). Otherwise, return <code>false</code>. e. If <code>Type(x)</code> is <code>Boolean</code>, return <code>true</code> if <code>x</code> and <code>y</code> are both <code>true</code> or both <code>false</code>. Otherwise, return <code>false</code>. f. Return <code>true</code> if <code>x</code> and <code>y</code> refer to the same object. Otherwise, return <code>false</code>.</li> <li>If <code>x</code> is null and <code>y</code> is undefined, return <code>true</code>. </li> <li>If <code>x</code> is undefined and <code>y</code> is null, return <code>true</code>. </li> <li>If <code>Type(x)</code> is <code>Number</code> and <code>Type(y)</code> is <code>String</code>, return the result of the comparison <code>x == ToNumber(y)</code>. </li> <li>If <code>Type(x)</code> is <code>String</code> and <code>Type(y)</code> is <code>Number</code>, return the result of the comparison <code>ToNumber(x) == y</code>. </li> <li>If <code>Type(x)</code> is <code>Boolean</code>, return the result of the comparison <code>ToNumber(x) == y</code>. </li> <li>If <code>Type(y)</code> is <code>Boolean</code>, return the result of the comparison <code>x == ToNumber(y)</code>. </li> <li>If <code>Type(x)</code> is either <code>String</code> or <code>Number</code> and <code>Type(y)</code> is <code>Object</code>, return the result of the comparison <code>x == ToPrimitive(y)</code>. </li> <li>If <code>Type(x)</code> is <code>Object</code> and <code>Type(y)</code> is either <code>String</code> or <code>Number</code>, return the result of the comparison <code>ToPrimitive(x) == y</code>. </li> <li>Return <code>false</code>.</li> </ol> </blockquote> Note that in the spec, the <code>Type</code> of a primitive string object is <code>String</code>, whereas the type of any object (including the <code>String</code> object) is <code>Object</code>. With <code>===</code> the relevant line is <code>#1</code>: the <code>Type</code> of the objects are different, so <code>false</code> is returned. With <code>==</code> the relevant line is <code>#8</code>: <code>x</code> is a <code>String</code> (<code>"Hello world!"</code>) and <code>y</code> is an <code>Object</code> (The <code>String</code> object containing the string <code>"Hello world!"</code>). Thus the comparison <code>x == ToPrimitive(y)</code> is made. <code>ToPrimitive</code> ends up calling the <code>valueOf</code> method of the object, or if that method doesn't exist, the <code>toString</code> method. In this case, a <code>String</code> object's <code>valueOf</code> method returns the primitive <code>string</code> the object contains. Thus the equality operation is done again, this time between two primitive <code>string</code>s which contain the same text, which returns <code>true</code> thanks to <code>#1.d</code>. JavaScript is a bit messy under the hood... <hr> EDIT: Notice that if two objects are compared, no conversions apply, but rather, rule <code>#1.f</code> applies. Thus, thanks to the spec, I was able to correctly predict the output of the following code: <pre><code>> new String("hi") == new String("hi") false </code></pre> <hr> EDIT: Just thought I'd add that these distinctions are even further blurred by more implicit type conversion. For example, the following works: <pre><code>> ("hi").toString() "hi" </code></pre> but that's not because <code>"hi"</code> is an object (like in Python): <pre><code>> typeof "hi" "string" </code></pre> But rather, because the <a href="http://ecma-international.org/ecma-262/5.1/#sec-11.2.1" rel="nofollow noreferrer"><code>.</code> operator</a> <a href="http://ecma-international.org/ecma-262/5.1/#sec-8.7.1" rel="nofollow noreferrer">does a conversion</a> from the primitive <code>string</code> type to the string <code>Object</code> type (creating a new string object) whose <code>toString</code> method is then called.
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