Try to truncate string substring in JS but did not work

Question

 function truncate(str, num) {
    if (str.length >= num) {
        if (num > 3) {
            str.substr(0, num - 3) + '...';
        } else {
            str.substr(0, num) + '...';
       }
    }   return str;
 }
console.log(truncate("A-tisket a-tasket A green and yellow basket", 11));

It returned me the original string A-tisket a-tasket A green and yellow basket. I wonder waht's the problem here.

Never mind, I did not invoke the substr method. Should be"str=str.substr(0, num - 3) + '...';" — Xinyi Li
– Xinyi Li, Commented Mar 18, 2018 at 14:13

SirPeople · Accepted Answer · 2018-03-18 14:17:12Z

1

substr is an immutable operation. That means that when you execute it, it doesnt change the original value of the string that you applied it to.

In order to make it work, you should save the substring in an additional variable:

function truncate(str, num) {
    if (str.length >= num) {

        if (num > 3) {
           return str.substr(0, num - 3) + '...';
        } else {
           return str.substr(0, num) + '...';
       }
    }   return str;
 }
console.log(truncate("A-tisket a-tasket A green and yellow basket", 11));

answered Mar 18, 2018 at 14:17

SirPeople

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Comments

brk · Accepted Answer · 2018-03-18 14:13:12Z

0

This is because the return str is pointing the str in the function argument.Create a separate variable and assign the substring to it and return the value

function truncate(str, num) {
  let subStr = '';
  if (str.length >= num) {
    if (num > 3) {
      subStr = str.substr(0, num - 3) + '...';
    } else {
      subStr = str.substr(0, num) + '...';
    }
  }
  return subStr;
}
console.log(truncate("A-tisket a-tasket A green and yellow basket", 11));

answered Mar 18, 2018 at 14:13

brk

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