blocks|key|1319906|text|if语句使用condition+==+true。它是在ECMAScript语言规范中给出的，在这里：http://www.ecma-international.org/ecma-262/5.1/#sec-12.5|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1319907|注意步骤2中ToBoolean()的使用，这会将给定的参数转换为布尔值，这意味着if语句的条件确实会发生类型强制。|1319908|entityMap|0|LINK|mutability|MUTABLE|url|http://www.ecma-international.org/ecma-262/5.1/#sec-12.5^0|0|2|6|H|1E|1K|0|0|6|B|14|2|0^^$0|@$1|2|3|4|5|6|7|P|8|@$9|Q|A|R|B|C]|$9|S|A|T|B|C]]|D|@$9|U|A|V|1|W]]|E|$]]|$1|F|3|G|5|6|7|X|8|@$9|Y|A|Z|B|C]|$9|10|A|11|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|12|8|@]|D|@]|E|$]]]|I|$J|$5|K|L|M|E|$N|O]]]]

The <code>if</code> statement uses <code>condition == true</code>. It's given in the ECMAScript Language Specification, here: <a href="http://www.ecma-international.org/ecma-262/5.1/#sec-12.5" rel="noreferrer">http://www.ecma-international.org/ecma-262/5.1/#sec-12.5</a>

Note the usage of the <code>ToBoolean()</code> in Step 2. This converts the given argument to a Boolean value, meaning that yes, type coercion does occur for the condition of an <code>if</code> statement.

blocks|key|1319919|text|Javascript+a.k.a.ECMAScript不是由W3C管理，而是由W3C管理。您可以阅读规范这里。您感兴趣的是这部分，它指定如何处理if语句中的表达式。运行时应该对表达式值调用toBoolean；这意味着任何类型都将转换为布尔值。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1319920|因此，它的行为类似于==。|1319921|entityMap|0|LINK|mutability|MUTABLE|url|http://www.ecma-international.org/ecma-262/5.1/|1|http://www.ecma-international.org/ecma-262/5.1/#sec-12.5^0|20|2|2M|9|1G|2|0|1P|1|1|0|A|2|0^^$0|@$1|2|3|4|5|6|7|R|8|@$9|S|A|T|B|C]|$9|U|A|V|B|C]]|D|@$9|W|A|X|1|Y]|$9|Z|A|10|1|11]]|E|$]]|$1|F|3|G|5|6|7|12|8|@$9|13|A|14|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|15|8|@]|D|@]|E|$]]]|I|$J|$5|K|L|M|E|$N|O]]|P|$5|K|L|M|E|$N|Q]]]]

Javascript a.k.a. ECMAScript is not governed by the W3C but by ECMA. You can read the specification <a href="http://www.ecma-international.org/ecma-262/5.1/" rel="noreferrer">here</a>. What you're interested in is <a href="http://www.ecma-international.org/ecma-262/5.1/#sec-12.5" rel="noreferrer">this</a> section, which specifies how expressions in <code>if</code> statements are handled. The runtime should call <code>toBoolean</code> on the expression value; meaning any type is going to be converted to a boolean.

Therefore it behaves like <code>==</code>.

blocks|key|2640360|text|因为===是严格的比较运算符。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2640361|if+(1+===+true)+{+//they+are+of+a+different+type|code-block|syntax|javascript|2640362|你试过这个吗？|2640363|if+(0)+{++//+This+fails
++++console.log("1");
}|2640364|因为0+=+off,+negative,+no,+false普遍存在。检查这个为什么0是假的？|2640365|而当你使用===的时候|2640366|if(+1+===+true)++//It+looks+for+type+1+as+number+and+true+as+boolean|2640367|ES5规范定义了以下算法：|2640368|​|2640369|📷|atomic|2640370|2640371|entityMap|0|LINK|mutability|MUTABLE|url|https://softwareengineering.stackexchange.com/questions/198284/why-is-0-false|1|http://javascriptweblog.wordpress.com/2011/02/07/truth-equality-and-javascript/|2|IMAGE|IMMUTABLE|imageUrl|https://developer.qcloudimg.com/http-save/yehe-900000/f5ef69c81931b92f790b78441f513920.png|imageAlt^0|2|3|0|0|0|0|2|S|13|8|0|0|5|3|0|0|0|D|1|0|0|0|1|2|0|0^^$0|@$1|2|3|4|5|6|7|1I|8|@$9|1J|A|1K|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|1L|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|1M|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|1N|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|1O|8|@$9|1P|A|1Q|B|C]]|D|@$9|1R|A|1S|1|1T]]|E|$]]|$1|Q|3|R|5|6|7|1U|8|@$9|1V|A|1W|B|C]]|D|@]|E|$]]|$1|S|3|T|5|H|7|1X|8|@]|D|@]|E|$I|J]]|$1|U|3|V|5|6|7|1Y|8|@]|D|@$9|1Z|A|20|1|21]]|E|$]]|$1|W|3|X|5|6|7|22|8|@]|D|@]|E|$]]|$1|Y|3|Z|5|10|7|23|8|@]|D|@$9|24|A|25|1|26]]|E|$]]|$1|11|3|X|5|6|7|27|8|@]|D|@]|E|$]]|$1|12|3|-4|5|6|7|28|8|@]|D|@]|E|$]]]|13|$14|$5|15|16|17|E|$18|19]]|1A|$5|15|16|17|E|$18|1B]]|1C|$5|1D|16|1E|E|$1F|1G|1H|-4]]]]

Because <code>===</code> is strict comparison operator.

<pre><code>if (1 === true) { //they are of a different type
</code></pre>

Have you tried this

<pre><code>if (0) { // This fails
 console.log("1");
}
</code></pre>

because <code>0 = off, negative, no, false</code> universally. check this <a href="https://softwareengineering.stackexchange.com/questions/198284/why-is-0-false">Why is 0 false?</a>

whereas whereas when you use <code>===</code> 

<pre><code>if( 1 === true) //It looks for type 1 as number and true as boolean
</code></pre>

<a href="http://javascriptweblog.wordpress.com/2011/02/07/truth-equality-and-javascript/" rel="nofollow noreferrer">ES5 spec defines the following algorithm:</a>

<img src="https://i.stack.imgur.com/hvPfO.png" alt="enter image description here">

blocks|key|242348|text|当您从一本关于CSS的书中获得这些信息时，作者很可能会引用如下CSS规则：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|242349|[if+IE]body
{/*+only+for+IE*/
++++[if+IE+6]+background:+url('get_real.gif');
++++[if+gt+IE+6]+background:+url('nogood_browser.png');
}
[if+Webkit]body
{/*Only+webkit+browsers+get+this+bg*/
++++background:+url('webkit_bg.png');
}
[if+Gecko]body
{/*Gecko-based+browsers+get+this*/
+++background:+url('foxy.png');
}|code-block|syntax|javascript|242350|除此之外，在JS上：|242351|我猜你会想到所有的错误信息都来自于这个主意。拜托，为了你和我的缘故，不要使用w3schools。|offset|length|242352|对于if+(condition)，condition应该读取expression才是正确的，如果表达式由单个值组成，则应该是真实的。如果您正在比较2个表达式，则条件表达式根据比较两个操作数的方式计算为true或false。|style|CODE|242353|单操作数：|242354|if+(1)//truthy
if+(undefined)+//falsy|242355|这是因为Boolean(1)+===+true和Boolean(undefined)+===+false|242356|一旦引入第二个操作数：|242357|if+('0'+==+0)//===+true
if+('0'+===+0)//+===+false|242358|这是因为===是一种类型和值检查，您显然已经知道了。只是为了不引起混淆：|242359|如果您想确保您有正确的信息:检查ECMAScript语言规范，而不是第三方资源：这是链接|242360|entityMap|0|LINK|mutability|MUTABLE|url|http://www.w3schools.com/js/js_if_else.asp|1|http://www.w3fools.com/|2|http://www.ecma-international.org/ecma-262/5.1/^0|0|0|0|6|B|0|Y|D|1|0|2|E|H|9|U|A|2S|4|2X|5|0|0|0|4|J|O|S|0|0|0|4|3|0|14|4|2|0^^$0|@$1|2|3|4|5|6|7|1G|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|1H|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|1I|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|1J|8|@]|9|@$K|1K|L|1L|1|1M]|$K|1N|L|1O|1|1P]]|A|$]]|$1|M|3|N|5|6|7|1Q|8|@$K|1R|L|1S|O|P]|$K|1T|L|1U|O|P]|$K|1V|L|1W|O|P]|$K|1X|L|1Y|O|P]|$K|1Z|L|20|O|P]]|9|@]|A|$]]|$1|Q|3|R|5|6|7|21|8|@]|9|@]|A|$]]|$1|S|3|T|5|D|7|22|8|@]|9|@]|A|$E|F]]|$1|U|3|V|5|6|7|23|8|@$K|24|L|25|O|P]|$K|26|L|27|O|P]]|9|@]|A|$]]|$1|W|3|X|5|6|7|28|8|@]|9|@]|A|$]]|$1|Y|3|Z|5|D|7|29|8|@]|9|@]|A|$E|F]]|$1|10|3|11|5|6|7|2A|8|@$K|2B|L|2C|O|P]]|9|@]|A|$]]|$1|12|3|13|5|6|7|2D|8|@]|9|@$K|2E|L|2F|1|2G]]|A|$]]|$1|14|3|-4|5|6|7|2H|8|@]|9|@]|A|$]]]|15|$16|$5|17|18|19|A|$1A|1B]]|1C|$5|17|18|19|A|$1A|1D]]|1E|$5|17|18|19|A|$1A|1F]]]]

As you're getting this info from a book on CSS, the author is most likely referring to CSS rules like this:

<pre><code>[if IE]body
{/* only for IE*/
 [if IE 6] background: url('get_real.gif');
 [if gt IE 6] background: url('nogood_browser.png');
}
[if Webkit]body
{/*Only webkit browsers get this bg*/
 background: url('webkit_bg.png');
}
[if Gecko]body
{/*Gecko-based browsers get this*/
 background: url('foxy.png');
}
</code></pre>

Other than that - On JS:

My guess would be you got the idea <a href="http://www.w3schools.com/js/js_if_else.asp" rel="nofollow">from the place all mis-information comes from</a>. Please, for your sake and mine, <a href="http://www.w3fools.com" rel="nofollow">don't use w3schools</a>.

in case of an <code>if (condition)</code>, where <code>condition</code> should read <code>expression</code> to be correct, it should be truthy, if the expression consists of a single value. In case you are comparing 2 expressions, the conditional expression evaluates to <code>true</code> or <code>false</code>, based on how you compare the two operands

Single operand:

<pre><code>if (1)//truthy
if (undefined) //falsy
</code></pre>

This is because <code>Boolean(1) === true</code> and <code>Boolean(undefined) === false</code> 
Once you introduce a second operand:

<pre><code>if ('0' == 0)//=== true
if ('0' === 0)// === false
</code></pre>

That's because <code>===</code> is a type and value check, which you obviously know already. Just so there's no confusion:

If you want to be sure you have the correct information: check the ECMAScript language specs, not some third party resource: <a href="http://www.ecma-international.org/ecma-262/5.1/" rel="nofollow">here's the link</a>

blocks|key|2640287|text|它与由condition+==+true语言规范指定的ECMAScript等效|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2640288|entityMap^0|3|H|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|K|8|@]|D|@]|E|$]]]|G|$]]

It is equivalent to <code>condition == true</code>, which is specified by ECMAScript Language Specification

blocks|key|242209|text|这三个等号的意思是“没有类型强制的平等”。使用三重等于，值的类型也必须相等。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|242210|==与===在JavaScript中的差异|offset|length|242211|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/523643/difference-between-and-in-javascript^0|0|0|L|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|O|8|@]|9|@$D|P|E|Q|1|R]]|A|$]]|$1|F|3|-4|5|6|7|S|8|@]|9|@]|A|$]]]|G|$H|$5|I|J|K|A|$L|M]]]]

The 3 equal signs mean "equality without type coercion". Using the triple equals, the values must be equal in type as well.

<a href="https://stackoverflow.com/questions/523643/difference-between-and-in-javascript">Difference between == and === in JavaScript</a>

blocks|key|2640474|text|特鲁西:评估为真的东西。福西:评估为假的东西。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2640475|1是真实的，0是假的。|2640476|任何类型的对象(包括函数、数组)都是真实的。|2640477|这些都是错误的:未定义的、null、NaN、0、"“(空字符串)和false。|2640478|因此，在第一个条件下，if+(1)...日志1，因为1是真实的。|offset|length|style|CODE|2640479|===专门测试类型相等，这与测试真实性不同。|2640480|entityMap^0|0|0|0|0|B|9|0|0^^$0|@$1|2|3|4|5|6|7|R|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|S|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|T|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|U|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|V|8|@$J|W|K|X|L|M]]|9|@]|A|$]]|$1|N|3|O|5|6|7|Y|8|@]|9|@]|A|$]]|$1|P|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|Q|$]]

Truthy: Something which evaluates to TRUE.
Falsey: Something which evaluates to FALSE.

1 is truthy, 0 is falsey. 

An object of any kind (including functions, arrays) is always truthy. 

These are all falsy: undefined, null, NaN, 0, "" (empty string), and false.

Hence, in your first condition <code>if (1)...</code> logs 1 because 1 is truthy.

=== specifically tests for type equality, which is different from testing truthy.

blocks|key|1320017|text|我在关于ToBoolean的ECMA文档中找到了该部分，该部分在if语句中使用。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1320018|概括如下：|1320019|数字:如果参数为%2B0、NaN或−0，则结果为false；否则结果为真。|blockquote|1320020|entityMap|0|LINK|mutability|MUTABLE|url|http://www.ecma-international.org/ecma-262/5.1/#sec-9.2^0|4|9|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@$A|R|B|S|1|T]]|C|$]]|$1|D|3|E|5|6|7|U|8|@]|9|@]|C|$]]|$1|F|3|G|5|H|7|V|8|@]|9|@]|C|$]]|$1|I|3|-4|5|6|7|W|8|@]|9|@]|C|$]]]|J|$K|$5|L|M|N|C|$O|P]]]]

I found the section in the ECMA documentation on <a href="http://www.ecma-international.org/ecma-262/5.1/#sec-9.2" rel="nofollow">ToBoolean</a>, which is used in the if statement.

To summarise:

<blockquote>
 Number : The result is false if the argument is +0, −0, or NaN;
 otherwise the result is true.
</blockquote>

I know the difference between <code>==</code> and <code>===</code> however I always believed that <code>if (condition)</code> <code>condition</code> was supposed to be evaluated against <code>true</code> using strict equality (<code>===</code>) and not type-coercing equality (<code>==</code>).

See an example:

<pre><code>if (1) {
 console.log("1");
}

if (1 == true) {
 console.log("2");
}

if (1 === true) {
 console.log("3");
}
</code></pre>

It returns:

<pre><code>::1
::2
</code></pre>

I know 1 is not strictly equal to <code>true</code>, because the type is different, but when I do <code>if (condition)</code> according to W3C it should be the strict equality test (<code>===</code>) that is run not the type-coercing equality of <code>==</code>.

So why is it logging 1?

In JS if (condition) means == true or === true

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我知道==和===的区别，但是我一直认为if (condition) condition应该使用严格等式(===)而不是类型胁迫等式(==)来评估true。见一个例子：if (1) {    console.log("1");}if (1 == true) {    console.log("2");}if (1 === true) {    console.log("3");}它返回：::1::

问在JS中，if (条件)是指== true或=== true
EN

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问在JS中，if (条件)是指== true或=== true
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