try this:

key=key.replace(/ /g,"_");

that'll do a global find/replace

javascript replace

To answer Prasanna's question below:

How do you replace multiple spaces by single space in Javascript ?

You would use the same function replace with a different regular expression. The expression for whitespace is \s and the expression for "1 or more times" is + the plus sign, so you'd just replace Adam's answer with the following:

key=key.replace(/\s+/g,"_");

You can try this

 var str = 'hello     world  !!';
 str = str.replace(/\s+/g, '-');

It will even replace multiple spaces with single '-'.

Replace spaces with underscore

var str = 'How are you';
var replaced = str.split(' ').join('_');

Output: How_are_you

I created JS performance test for it http://jsperf.com/split-and-join-vs-replace2

Replace all occurrences

This is happening because replace() method is designed this way to replace only the first occurance when you use string to find the match. Check the replace method.

To replace all matches you can use the following 3 methods:

1. use regex with the global flag in replace() method:

   When you use the replace method with regex with /g flag it replaces all the matching occurrences in a string.

       function updateKey()
       {
           var key=$("#title").val();
           key=key.replace(/ /g,"_");
           $("#url_key").val(key);
       }
       // Show case
       let title = "Your document title";
       console.log(title.replace(/ /g,"_"));

2. Using replaceAll method:

   The replaceAll method will remove all spaces with an underscore. (must use the global flag with it when using regex)

       function updateKey()
       {
           var key=$("#title").val();
           key=key.replaceAll(/ /g,"_");
           // key=key.replaceAll(" ","_"); also valid
           $("#url_key").val(key);
       }
       // Show case
       let title = "Your document title";
       console.log(title.replaceAll(/ /g,"_"));

3. Use a combination of split and join method:

   Split your string at spaces and join it by using _ as a separator in the join method.

       function updateKey()
       {
           var key=$("#title").val();
           key=key.split(" ").join("_");
           $("#url_key").val(key);
       }
       // Show case
       let title = "Your document title";
       console.log(title.split(" ").join("_"));

Just using replace:

var text = 'Hello World';
    
new_text = text.replaceAll(' ', '_');
    
console.log(new_text);

I know this is old but I didn't see anyone mention extending the String prototype.

String.prototype.replaceAll = function(search, replace){
    if(!search || !replace){return this;} //if search entry or replace entry empty return the string
    return this.replace(new RegExp('[' + search + ']', 'g'), replace); //global RegEx search for all instances ("g") of your search entry and replace them all.
};

$(function() {
    $('#title').keyup(function() {
        var value = $(this).val().toLowerCase();
        $('#url_key').val(value.split(' ').join('_'));
    }).keyup();
});

-- or you can use:

$(function() {
    $('#title').keyup(function() {
        var value = $(this).val().toLowerCase();
        $('#url_key').val(value.replace(/ /g,"_"));
    }).keyup();
});