55 lines
2.5 KiB
Markdown
55 lines
2.5 KiB
Markdown
---
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title: JavaScript Comparison operation at a glance
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date: 2019-03-28 09:00:00
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tags:
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- JavaScript
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category: tech
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keywords:
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- Javascript
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- ES2015
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- ES6
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---
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When given a scenario like:
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```javascript
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console.log(null > -1) //true
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```
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It produces `true`, which makes me think `null` is treated as `0`. But when I run:
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```javascript
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console.log(null == 0) // false
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console.log(null > 0) // false
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console.log(null < 0) // false
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```
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They all output `false`!
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I googled a lot and finally found answers in [Ecma-262 Specification](http://www.ecma-international.org/ecma-262/8.0/#sec-abstract-equality-comparison).
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The comparison `x == y`, where x and y are values, produces true or false. Such a comparison is performed as follows:
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<!-- more -->
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```text
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1. If Type(x) is the same as Type(y), then return the result of performing Strict Equality Comparison x === y.
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2. If x is null and y is undefined, return true.
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3. If x is undefined and y is null, return true.
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4. If Type(x) is Number and Type(y) is String, return the result of the comparison x == ToNumber(y).
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5. If Type(x) is String and Type(y) is Number, return the result of the comparison ToNumber(x) == y.
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6. If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.
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7. If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).
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8. If Type(x) is either String, Number, or Symbol and Type(y) is Object, return the result of the comparison x == ToPrimitive(y).
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9. If Type(x) is Object and Type(y) is either String, Number, or Symbol, return the result of the comparison ToPrimitive(x) == y.
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10. Return false.
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```
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Relational comparison is much more complex so I'm not copying that section. Read at the [spec website](http://www.ecma-international.org/ecma-262/8.0/#sec-abstract-relational-comparison).
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## TL;DR
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Anyway it seems that in `null == 0`, `null` is treated just as is, and **equality** comparison between `null` and `Number` always return **false** (No 10).
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But when it comes `null > -1`, `null` is conversed to 0 using [ToNumber()](http://www.ecma-international.org/ecma-262/8.0/#sec-tonumber) algorithm.
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Read more:
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* [https://github.com/getify/You-Dont-Know-JS/issues/1238](https://github.com/getify/You-Dont-Know-JS/issues/1238)
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* [http://www.ecma-international.org/ecma-262/8.0/#sec-abstract-relational-comparison](http://www.ecma-international.org/ecma-262/8.0/#sec-abstract-relational-comparison)
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* [http://www.ecma-international.org/ecma-262/8.0/#sec-tonumber](http://www.ecma-international.org/ecma-262/8.0/#sec-tonumber)
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