冗余 if 语句？

Question

Is the if statement in the following example redundant?

if (str[0] === ' ') {
  str = str.trimStart()
}

It seems unnecessary to me since trimStart() does not seem to have any negative effect on a string without a space at the beginning. Additionally, if the string does have a space at the beginning, you are running twice as many operations as needed.

Is the if statement ever necessary in situations like the one above?

Answer 1

Given the edited code, there's one situation where the if statement could be useful - if the string starts with a newline (which counts as whitespace, and would be trimmed), not a plain space. For example:

 const parseStr = (str) => { if (str[0] === ' ') { str = str.trimStart() } console.log(str.length); }; parseStr('\\nfoo'); parseStr(' foo'); parseStr('foo');

If trimStart was called unconditionally, the newline would be trimmed regardless:

 const parseStr = (str) => { str = str.trimStart() console.log(str.length); }; parseStr('\\nfoo'); parseStr(' foo'); parseStr('foo');

Answer 2

The if is not necessary, but you should save the trimmed value somewhere, like

str = str.trimStart();

-- Edit --

"Is the 'if' ever necessary...?"

The test that you are doing in the original is redundant. But you might want to check that str has a value and/or check that this value is a string before calling trimStart(), like

if (str && typeof str === 'string') ...

Answer 3

The if block isn't necessary since the trimStart function won't have any effect if there is not space in the beginning of the string. Having the if statement does not have any effect over it and wouldn't really have any real performance benefits either.