使用|| 在javascript中执行null / undefined检查？

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

Answer 1

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" 

Answer 2

Using a || b 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;

Answer 3

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.