如果整个表达式为真，javascript 输出非布尔值的行为是什么

Question

I know this question has been asked for before, but I can't seem to find the right keywords to find my answer.

In javascript, you can do this

console.log(true && "hello world"); // prints hello world
console.log(false && "hello world"); // prints false

What is this behaviour called? Where javascript will output the contents of a variable if the entire expression evaluates to true?

Answer 1

In the React world, This is called Conditional Rendering using the && operator. This technique leverages how Javascript short-circuits comparisons to change the view; in this case, a logged value. It is a concise and useful way to dynamically render different objects as properties change.

If you weren't rendering, it would just be a good old-fashioned short-circuit, which has been used for decades, and goes all the way back to Kernaghan and Ritchie and C language. Eg the OR operator can be used when referencing object properties to guard against undefined values; eg:

const cheese = {}
console.log(cheese.cheddar) // unsafe
console.log(cheese.cheddar || "") // safe

Here, we don't really care about the actual value, we just need a truthy or falsy resolution.

Also note the double-bang !! operator, which will convert the whole darn expression to a boolean:

console.log(!!(true && "hello world")) // true

References: https://reactjs.org/docs/conditional-rendering.html https://codeburst.io/javascript-short-circuit-conditionals-bbc13ac3e9eb What is the?! (not not) operator in JavaScript?