React showing 0 instead of nothing with short-circuit (&&) conditional component

问题

I have the following simple short-circuit statement that should show either a component or nothing:

{profileTypesLoading && <GeneralLoader />}

If the statement is false, it renders a 0 instead of nothing.

I have done a console.log(profileTypesLoading) just to see quickly what the status of the profileTypesLoading property is and it's either 1 or 0 as expected. 0 should be false... causing nothing to render. Right?

Any idea why this would happen?

回答1:

Since your condition is falsy and so doesn't return the second argument (<GeneralLoader />), it will return profileTypesLoading, which is a number, so react will render it because React skips rendering for anything that is typeof boolean or undefined and will render anything that is typeof string or number:

To make it safe, you can either use a ternary expression {condition ? <Component /> : null} or boolean cast your condition like {!!condition && <Component />}

回答2:

0 is a falsy value, so when it is evaluated by &&, it returns 0. However, 0 is renderable by React because it is a number:

// Renderable values
1 && <GeneralLoader /> // => Renders <GeneralLoader />
"a string" && <GeneralLoader /> // => Renders <GeneralLoader />
0 && <GeneralLoader /> // => Renders '0'

// Non-renderable values
false && <GeneralLoader /> // => Renders nothing
null && <GeneralLoader /> // => Renders nothing
undefined && <GeneralLoader /> // => Renders nothing

TLDR

This is because of how javascript itself process [truthy and falsy values][1]:

In JavaScript, a truthy value is a value that is considered true when encountered in a Boolean context. All values are truthy unless they are defined as falsy (i.e., except for false, 0, "", null, undefined, and NaN).

When used with the && operator, the returned value depends on the left value:

If the left value is truthy, the right value is returned.
If the left value is falsy, its value is returned.

Examples:

// Truthy values
1 && "hello" // => "hello"
"a string" && "hello" // => "hello"

// Falsy values
0 && "hello" // => 0
false && "hello" // => false
null && "hello" // => null
undefined && "hello" // => undefined

The same rules applies to JSX because it is [a syntax extension to JavaScript][2]. However, the issue is that **

The issue is that 0 is a falsy value, so when it is evaluated by &&, it returns 0. However, 0 is renderable by React because it is a number

// Renderable values
1 && <GeneralLoader /> // => Renders <GeneralLoader />
"a string" && <GeneralLoader /> // => Renders <GeneralLoader />
0 && <GeneralLoader /> // => Renders 0

// Non-renderable values
false && <GeneralLoader /> // => Renders nothing
null && <GeneralLoader /> // => Renders nothing
undefined && <GeneralLoader /> // => Renders nothing

回答3:

This would solve the problem:

{!!profileTypesLoading && <GeneralLoader />}

As it will convert 0 to false. The reason is when it's 0 the next condition doesn't get executed and it behaves like a number in JavaScript so double negation helps here.

来源：https://stackoverflow.com/questions/53048037/react-showing-0-instead-of-nothing-with-short-circuit-conditional-component

标签

reactjs

components

short-circuiting