Replace special characters in a string with underscore (_) in JavaScript

JavaScript replace() method is used to replace all special characters from a string with _ (underscore) which is described below:

replace() method: This method searches a string for a defined value, or a regular expression, and returns a new string with the replaced defined value.

Syntax:
```
string.replace(searchVal, newvalue)
```
Parameters:
- searchVal: It is required parameter. It specifies the value, or regular expression, that is going to replace by the new value.
- newvalue: It is required parameter. It specifies the value to replace the search value with.
Return value: It returns a new string where the defines value has been replaced by the new value.

Example 1: This example replaces all special characters with _ (underscore) using replace() method.

<!DOCTYPE HTML>  
<html>  
    <head>  
        <title>  
            Replace special characters in a 
            string with _ (underscore) 
        </title> 
    </head>  
      
    <body style = "text-align:center;">  
      
        <h1 style = "color:green;" >  
            GeeksForGeeks  
        </h1>  
          
        <p id = "GFG_UP" style = "font-size: 16px; font-weight: bold;"> 
        </p> 
          
        <button onclick = "gfg_Run()">  
            Click here 
        </button> 
          
        <p id = "GFG_DOWN" style =  
            "color:green; font-size: 20px; font-weight: bold;"> 
        </p> 
          
        <script> 
            var el_up = document.getElementById("GFG_UP"); 
            var el_down = document.getElementById("GFG_DOWN"); 
            var str = "This, is# GeeksForGeeks!"; 
            el_up.innerHTML = str; 
              
            function gfg_Run() { 
                el_down.innerHTML =  
                    str.replace(/[&\/\\#, +()$~%.'":*?<>{}]/g, '_'); 
            }          
        </script>  
    </body>  
</html>                     

Output:

Before clicking on the button:
After clicking on the button:

Example 2: This example replace a unique special character with _ (underscore). This example goes to each character and checks if it is a special character that we are looking for, then it will replace the character. In this example the unique character is $(dollar sign).

<!DOCTYPE HTML>  
<html>  
    <head>  
        <title>  
            Replace special characters with 
            _ (underscore) 
        </title> 
    </head>  
      
    <body style = "text-align:center;">  
      
        <h1 style = "color:green;" >  
            GeeksForGeeks  
        </h1>  
      
        <p id = "GFG_UP" style = "font-size: 16px; font-weight: bold;"> 
        </p> 
          
        <button onclick = "gfg_Run()">  
            Click here 
        </button> 
          
        <p id = "GFG_DOWN" style =  
            "color:green; font-size: 20px; font-weight: bold;"> 
        </p> 
          
        <script> 
            var el_up = document.getElementById("GFG_UP"); 
            var el_down = document.getElementById("GFG_DOWN"); 
            var str = "A$computer$science$portal$for$Geeks"; 
            el_up.innerHTML = str; 
              
            function gfg_Run() { 
                  
                var newStr = ""; 
                  
                for(var i=0; i<str.length; i++) {  
                    if (str[i] == '$') { 
                        newStr += '_'; 
                    } 
                    else { 
                        newStr += str[i]; 
                    } 
                } 
                el_down.innerHTML = newStr; 
            }          
        </script>  
    </body>  
</html>                     

Output:

Before clicking on the button:
After clicking on the button:

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