Javascript内联if语句

Question

I had a bit of a weird result in the javascript console. I was trying to look for an alternative (more readable) version of the ternary operator, just for fun. Typing:

{ if(3===4) {5} else {6} }

Evaluates in my console to 6, but for some reason, I can not assign it to a variable, so running:

let a = { if(3===4) {5} else {6} }

Does not let me store it to the variable directly. So my main question is, if this block is returning something, why can't I assign it?

Answer 1

The fact that blocks (and other statements) return values isn't accessible to your code. Consoles can see the result, and it exists at a language specification level, but not in your code.

Your options are the conditional operator¹ (which is quite readable when you're used to it, but you've said you're looking for alternatives to it) or the if / else assigning to a in both parts:

let a;
if (3 === 4) {
    a = 5;
} else {
    a = 6;
}

Or you could use an inline function (IIFE):

let a = (() => { if (3 === 4} return 5 else return 6; })();

There is also a proposal being floated for " do expressions ", which would look like this:

// Currently a proposal, not implemented in engines yet
let a = do { if (3 === 4) 5; else 6; };

That proposal is at Stage 1 of the process , so it may or may not progress, and if it progresses it could change markedly before doing so.

---

¹ Although you see "the ternary operator" a lot, the proper name is the conditional operator. It is a ternary operator (an operator accepting three operands), and currently JavaScript's only ternary operator, but that could change someday. :-)

Answer 2

使用三元运算符 ，因为你不能分配if语句：

let a = 3 === 4 ? 5 : 6;

Answer 3

The reason this doesn't work is because if is, as you pointed out, a statement. An assignment requires an expression holding a value to be assigned. A statement doesn't ( per se ) have a value--it simply performs some side-effect instead.

What you can use if you want to assign a value conditionally is the ternary conditional operator, which is an expression:

let a = (3 === 4 ? 5 : 6)

At this point you're probably wondering, why does the console print a value if a statement merely performs side-effects? This is because statements have a completion value , which for an if statement is the completion value of the corresponding branch, and for an expression statement (which is what the 5 and 6 inside the blocks are), is whatever the expression evaluates to. This is mainly useful for when you want to evaluate an expression in the console and see what it produces, without having to issue a console.log() each time you do.

Answer 4

It's not returning anything. if is not an expression, it's a statement; it has no value.

The point of the ternary operator was to provide a more readable version of this:

let a;
if (condition) {
    a = 1;
}
else {
    a = 2;
}