代码中条件表达式内赋值操作的执行逻辑解析

Hey there, let's break down both of your questions clearly, step by step!

First, let's recall two key JavaScript behaviors that make this line tick:

• Assignment expressions have a return value: When you run x = y, the entire expression evaluates to y (the value being assigned to x).
• Operator precedence matters: The parentheses around value = f ? f(prop) : prop are critical—they ensure the ternary operator runs first, then the assignment, before the && checks the result.

Let's unpack the full condition line by line:

if (prop != null && (value = f ? f(prop) : prop))

Here's the play-by-play:

1. First, we check if prop != null (meaning prop isn’t null or undefined). If this is false, the condition short-circuits immediately, and we skip the return block.
2. If prop is valid, we run the ternary operator f ? f(prop) : prop:
   • If f is a "truthy" value (not null, undefined, false, 0, empty string, etc.), we call f(prop) and use its return value.
   • If f is "falsy", we just use the raw prop value directly.
3. We assign that ternary result to value.
4. The entire value = ... expression returns the value we just assigned to value. We then check if that value is truthy—if yes, we return value right away.

In short, this line does two things in one go: it computes and stores a value in value, and checks if that computed value is truthy to decide whether to return it.

Let's revisit your example code:

let f = 42; 
value = f ? console.log('true') : console.log('false'); 
console.log(value);

The root of this is understanding what console.log() returns.

When you run console.log('true'), it prints the string to your console—but the function itself returns undefined. JavaScript functions without an explicit return statement default to returning undefined, and that's exactly what console.log does.

Since f = 42 is truthy, the ternary operator picks the first branch: console.log('true'). The result of that branch is undefined, which gets assigned to value. So when you log value, you see undefined.

To confirm this, try swapping console.log for a function that returns a concrete value:

let f = 42;
value = f ? (() => 'I am a real return value')() : (() => 'Falsy case result')();
console.log(value); // Outputs: "I am a real return value"

Here, the arrow functions explicitly return strings, so value gets that string instead of undefined.

内容的提问来源于stack exchange，提问作者webstackdev