简写 if/else 语句 Javascript

Question

I'm wondering if there's a shorter way to write this:

var x = 1;
if(y != undefined) x = y;

I initially tried x = y || 1 x = y || 1 , but that didn't work. What's the correct way to go about this?

Answer 1

var x = y !== undefined ? y : 1;

Note that var x = y || 1; var x = y || 1; would assign 1 for any case where y is falsy (eg false , 0 , "" ), which may be why it "didn't work" for you. Also, if y is a global variable, if it's truly not defined you may run into an error unless you access it as window.y .

---

As vol7ron suggests in the comments, you can also use typeof to avoid the need to refer to global vars as window.<name> :

var x = typeof y != "undefined" ? y : 1;

Answer 2

Another way to write it shortly

bePlanVar = !!((bePlanVar == false));

// is equivalent to

bePlanVar = (bePlanVar == false) ? true : false;

// and 

if (bePlanVar == false) {
    bePlanVar = true;
} else {
    bePlanVar = false;
}

Answer 3

y = (y != undefined) ? y : x;

括号不是必需的，我只是添加它们，因为我认为这样更容易阅读。

Answer 4

Other way is using short-circuit:

x = (typeof y !== 'undefined') && y || 1

Although I myself think that ternary is more readable.

Answer 5

Here is a way to do it that works, but may not be best practise for any language really:

var x,y;
x='something';
y=1;
undefined === y || (x = y);

alternatively

undefined !== y && (x = y);

Answer 6

Appears you are having 'y' default to 1: An arrow function would be useful in 2020:

let x = (y = 1) => //insert operation with y here

Let 'x' be a function where 'y' is a parameter which would be assigned a default to '1' if it is some null or undefined value, then return some operation with y.

Answer 7

You can try if/else this shorthand method:

// Syntax
if condition || else condition

// Example
let oldStr = "";
let newStr = oldStr || "Updated Value";
console.log(newStr); // Updated Value

// Example 2
let num1 = 2;
let num2 = num1 || 3;
console.log(num2);  // 2  cause num1 is a truthy