How to replace specific characters within a string in JavaScript?

Question

Given a string like aaabba, I want a function which turns the a's to b's and the b's to a's, returning bbbaab.

I tried this clumsy one:

var newa = [];
function swit(x) { 
    for (var i = 0; i < x.length; i++) {
        if (x[i] === 'a') {
            newa.push('b');
        } 
        else if (x[i] === 'b') {
            newa.push('a');
        } 
        alert(newa);
    }
}
swit("aaab");

After clicking through a lot of alerts, finally, the last alert shows the intended result. But I want it without commas and at the first place – not after many iterations:

I also tried the string.replace() Method – in combination with a for-loop:

function swit(x) { 
    for (var i = 0; i < x.length; i++) {
        if (x[i] === 'a') {
            x.replace('a', 'b');
        } 
        else if (x[i] === 'b') {
            x.replace('b', 'a');
        } 
        alert(x);
    }
}
swit("aaab");

But nothing changes. Why?

Could you please give me a hint of what went wrong and how to get the desired result?

Well when you do an alert with an array it uses toString() which has commas.... — epascarello
– epascarello, Commented Oct 2, 2018 at 12:00
Have you though about using regular expressions? You can either use myString.replace(/somePattern/g, 'replaceTo') or with use of new RegExp(options). Playground here: regexr.com, my quick (may not work the way you want it to but you get the idea) example here: jsfiddle.net/b1hgtnfy/11 — user7820960
– user7820960, Commented Oct 2, 2018 at 12:08

Shyam Kumar · Accepted Answer · 2018-10-02 12:04:21Z

1

var str = "aaab";
str = str.replace(/a|b/g, v => {
       if(v=="a"){
         return "b"
       }else{
         return "a"
       }
    });

console.log(str);

answered Oct 2, 2018 at 12:04

Shyam Kumar

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1 Comment

dossy Over a year ago

You can eliminate the if/else tree (especially once you start adding more replacement rules) with a hash map: "aaabba".replace(/a|b/g, x => { const map = { a: 'b', b: 'a' }; return map[x]; })

ponury-kostek · Accepted Answer · 2018-10-02 12:15:12Z

1

Easiest way is to split text to single characters using split, then map array of chars by simply replace a => b and b => a and join it together again, or just use replace

'bbbaab'.split('').map(char => char === 'a' ? 'b' : 'a').join('');

'bbbaab'.replace(/a|b/g, (match) => match === 'a' ? 'b' : 'a');

function swap(str) {
  return str.split('').map(char => char === 'a' ? 'b' : 'a').join('');
}
function swap2(str) {
  return str.replace(/a|b/g, (match) => match === 'a' ? 'b' : 'a');
}

console.log(swap('bbbaab'));
console.log(swap2('bbbaab'));

edited Oct 2, 2018 at 12:15

answered Oct 2, 2018 at 11:58

ponury-kostek

8,0835 gold badges27 silver badges33 bronze badges

Comments

Sash Sinha · Accepted Answer · 2018-10-02 12:12:13Z

0

You need to move your alert line out of the for loop to the end, and use Array.join() to convert newa into a string without commas.

Also, it would make sense to move the newa array declaration inside the swit function.

Lastly, you should consider also adding an else condition where you just push the exact same current character onto the newa array if it is not an a or b, so the output string length is the same as the original string.

function swit(x) { 
  var newa = [];
  for (var i = 0; i < x.length; i++) {
    if (x[i] === 'a') {
      newa.push('b');
    } else if (x[i] === 'b') {
      newa.push('a');
    } else {
      newa.push(x[i])
    }
  }
  alert(newa.join(""));
}

swit("aaab");
swit("aasdfcvbab");

edited Oct 2, 2018 at 12:12

answered Oct 2, 2018 at 12:04

Sash Sinha

22.8k4 gold badges27 silver badges48 bronze badges

Comments

nbokmans · Accepted Answer · 2018-10-02 11:57:47Z

Your first example works but replaces a string with an array of characters (which is different from a string). You can simply join the resulting array to get desired result:

function swit(x) {
    var newa = [];

    for (var i = 0; i < x.length; i++) {
        if (x[i] === 'a') {
            newa.push('b');
        } else if (x[i] === 'b') {
            newa.push('a');
        }
    }

    var arrayJoined = newa.join('');
    alert(arrayJoined);
    return arrayJoined;
}

For your second example, replace replaces all occurences in a string and does not need to be run in a for loop. So what you actually do in your second code sample is: for each character in string: if character is 'a', replace by b. if character is 'b', replace by a. Which basically just switches all a's to b's and b's back to a's. Which results in the same output string as your input string.

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