JavaScript Standard Style

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* **複数行の三項演算子では、`?`と`:`を各行に書いてください。**

eslint: [`operator-linebreak`](

// ✓ ok
var location = env.development ? 'localhost' : ''

// ✓ ok
var location = env.development
  ? 'localhost'
  : ''

// ✗ avoid
var location = env.development ?
  'localhost' :





"An Open Letter to JavaScript Leaders Regarding Semicolons" からの抜粋:

[Relying on automatic semicolon insertion] is quite safe, and perfectly valid JS that every browser understands. Closure compiler, yuicompressor, packer, and jsmin all can properly minify it. There is no performance impact anywhere.

I am sorry that, instead of educating you, the leaders in this language community have given you lies and fear. That was shameful. I recommend learning how statements in JS are actually terminated (and in which cases they are not terminated), so that you can write code that you find beautiful.

In general, \n ends a statement unless: 1. The statement has an unclosed paren, array literal, or object literal or ends in some other way that is not a valid way to end a statement. (For instance, ending with . or ,.) 2. The line is -- or ++ (in which case it will decrement/increment the next token.) 3. It is a for(), while(), do, if(), or else, and there is no { 4. The next line starts with [, (, +, *, /, -, ,, ., or some other binary operator that can only be found between two tokens in a single expression.

The first is pretty obvious. Even JSLint is ok with \n chars in JSON and parenthesized constructs, and with var statements that span multiple lines ending in ,.

The second is super weird. I’ve never seen a case (outside of these sorts of conversations) where you’d want to do write i\n++\nj, but, point of fact, that’s parsed as i; ++j, not i++; j.

The third is well understood, if generally despised. if (x)\ny() is equivalent to if (x) { y() }. The construct doesn’t end until it reaches either a block, or a statement.

; is a valid JavaScript statement, so if(x); is equivalent to if(x){} or, “If x, do nothing.” This is more commonly applied to loops where the loop check also is the update function. Unusual, but not unheard of.

The fourth is generally the fud-inducing “oh noes, you need semicolons!” case. But, as it turns out, it’s quite easy to prefix those lines with semicolons if you don’t mean them to be continuations of the previous line. For example, instead of this:


you could do this:


The advantage is that the prefixes are easier to notice, once you are accustomed to never seeing lines starting with ( or [ without semis.