JS replacing all occurrences of string using variable [duplicate]

The RegExp constructor takes a string and creates a regular expression out of it.

function name(str,replaceWhat,replaceTo){
    var re = new RegExp(replaceWhat, 'g');
    return str.replace(re,replaceTo);
}

If replaceWhat might contain characters that are special in regular expressions, you can do:

function name(str,replaceWhat,replaceTo){
    replaceWhat = replaceWhat.replace(/[-\/\\^$*+?.()|[\]{}]/g, '\\$&');
    var re = new RegExp(replaceWhat, 'g');
    return str.replace(re,replaceTo);
}

See Is there a RegExp.escape function in Javascript?