Using || to perform null / undefined check in javascript?

Question

I've read code that has snippets similar to this but I obviously forgot the semantics:

        let serve = target || "Hello World";

In other words if target is null, the serve equals Hello World. My goal, since target is null, is to get serve to be Hello Word ...

If I run the function as stated node prints this:

ReferenceError: target is not defined

As the error is trying to tell you, you cannot reference a variable that was never declared. — SLaks
– SLaks, Commented May 7, 2018 at 22:05
@Ole: If you want to be sure that you don't use variables you haven't declared, I'd recommend using eslint. You can set it up to show you errors for things like undeclared variables. Best to keep the code clean and avoid unnecessary checks :) — dani-sc
– dani-sc, Commented May 7, 2018 at 22:25

dani-sc · Accepted Answer · 2018-05-07 22:06:53Z

3

You need to define the variable target first. Here are some examples:

let target;
let serve = target || "Hello World";
console.log(serve); // prints "Hello World";

target = null;
serve = target || "Hello World";
console.log(serve); // still prints "Hello World";

target = "Cat";
serve = target || "Hello World";
console.log(serve); // prints "Cat"

answered May 7, 2018 at 22:06

dani-sc

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Comments

Sean · Accepted Answer · 2018-05-07 22:07:07Z

2

Using a || b will return b if a is falsy. The falsy values from You Don't Know JS: Types and Grammar - Chapter 4: Coercion are:

undefined
null
false
+0, -0, and NaN
""

If you'd like to return the default only when target is null, use:

let serve = target === null ? "Hello World" : target;

answered May 7, 2018 at 22:07

Sean

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Comments

Quentin · Accepted Answer · 2018-05-07 22:07:16Z

2

target, in your example is not null. It isn't anything: You haven't declared it at all.

let target = null;
let serve = target || "Hello World";
console.log(serve);

Possibly you are thinking of the pattern:

var serve = serve || "Hello World";
console.log(serve);

Which:

Uses var to ensure that serve is a declared variable
Assigns "Hello World" to serve is some previous code hasn't already assigned it a true value.

answered May 7, 2018 at 22:07

Quentin

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