Rule proposal: Prefer array.flat() over array.flat(1)

[jonahsnider]

Description

Array.prototype.flat uses 1 as the argument if you pass undefined or do not provide a value.
This means that .flat(1) is the same as .flat(), making the argument unnecessary.

Fail

array.flat(1);

const one = 1;
array.flat(one);

Pass

array.flat();

const one = 1;
array.flat();

let levels = 1; // Mutable variable, might be reassigned later
array.flat(levels);

[bmish]

Would it make sense to just add this check to the existing prefer-array-flat rule?

[sindresorhus]

Not sure it belongs in prefer-array-flat. That rule is not about call style.

Would maybe be a better fit for #1356?

[fisker]

We already have require-array-join-separator and require-number-to-fixed-digits-argument doing similar stuff, we can add this before we implement #1356.

[fisker]

I wonder if someone prefer use array.flat(1), it's explicit, if it does, we can add option for it.

[fregante]

All the rules mentioned here are a variation of "enforce or avoid the default value".

• avoids default: this suggestion
• enforces default: require-array-join-separator
• enforces default: require-number-to-fixed-digits-argument
• avoids default: no-useless-undefined

It almost feels like they belong together. I'm not particularly a fan of enforcing default values, but they do make it clearer sometimes.