Given a non whole number, or floating-point number (e.g. 3.5), what happens when using that number as an index?

When using a floating-point number to access an index of an array or string, undefined is returned.

'hello'[3.5]; // => undefined

When using floating-point numbers as indices passed to Array.prototype.slice() or String.prototype.slice() it seems the number is rounded down or the decimal is ignored.

['a','b','c','d','e'].slice(3.5, 4.5); // => ['d']
'hello'.slice(0.5, 1.5); // => 'h'

Why the different behavior, and what is actually happening here?

CodePudding user response：

.slice internally rounds its arguments to integers.

String.prototype.slice ( start, end )

1. Let O be ? RequireObjectCoercible(this value).
2. Let S be ? ToString(O).
3. Let len be the length of S.
4. Let intStart be ? ToIntegerOrInfinity(start).
5. If intStart is -∞, let from be 0.
6. Else if intStart < 0, let from be max(len intStart, 0).
7. Else, let from be min(intStart, len).
8. If end is undefined, let intEnd be len; else let intEnd be ? ToIntegerOrInfinity(end).
9. ...

where ToIntegerOrInfinity does

5. Let integer be floor(abs(ℝ(number))).

In contrast, when you do

'hello'[3.5];

no such rounding occurs.