if语句中有两组括号，这是什么意思？

Question

Recently I saw some weird code that looks kinda like this:

let a;
if ((a = anotherVariable)) {
 ....
}

Can you explain to me what does this mean and why is it done?

Edit:

Notice that he's not comparing a to anotherVariable , he's assigning it. And there are no other terms in the if parentheses, so it's not like if ((something) && (something else)) {...} .

So why not just assign a variable separately, and then check for truthiness of anotherVariable ?

Original code here (search for matched = prevTextFromSpace ).

Answer 1

If you write

if (a = anotherVariable) {
 ....
}

it works fine but = is an assignment, not a comparison.

The value of the expression a = b is the value of b (and it is also stored in the variable a ). If b evaluates to a truish value then the if branch is taken.

However, because = is not == , the linter programs usually flag an if statement as above as a potential error. It happens many times that the developer wanted == and typed = by mistake. The linters try to detect this error.

If the real intention is to copy the value of b in a and also use this value as the condition of the if statement (and not to compare a to b ) then the code above is correct. There are two ways to silent the linter warning:

1. Put the assignment in front of the if statement:

    a = b if (a) { ... } 

2. Put the assignment inside a pair of parentheses:

    if ((a = b)) { ... } 

   This decouples the assignment from the if . The assignment is not the if condition any more. The assignment is just a sub-expression of the larger expression (a = b) .

   This larger expression is the condition of the if statement and it is not an assignment, so there is nothing to warn about. Yes, it contains an assignment but the assignment is just a sub-expression.

It looks like a hack but it is not. It only denotes a good understanding of expressions and statements in programming languages.

Answer 2

The inner parenthesis simply evaluates the expression it contains. It will assign the value of anotherVariable to the variable a and that new value will be compared with the other operand in the condition.

 let a = 2, anotherVariable=20; if ((a = anotherVariable) == 20) { console.log(a) } 

Answer 3

It doesn't actually do anything except group the components. Let's say it looked like this:

if (a && (b || c))

Here, the parentheses would matter, because without them, JavaScript would interpret them incorrectly IIRC, as it'd look like this:

if (a && b || c)

And it's hard to tell what that'd output. Grouping is also important when you're using multiple ternary operators, like this:

if ((a : b ? c) && (d : g ? f))

Here, the parentheses are extremely important - bit of an extreme example, I know, but it illustrates my point nicely.

In short, parentheses grouping is a good habit to get into in JavaScript, as even when you have extremely simple conditions like:

if ((a > b) && (c < d))

It's very easy to get confused when you are meant to have two or more layers of parentheses. Imagine thinking through this with no parentheses!

if ((((a ? b : c) == (d < e)) && (f !== g)) || !(h == j))

Hopefully this helps!