console

1console.log("%c%s",
2            "color: red; background: yellow; font-size: 24px;",
3            "WARNING!");

1console.log('hello')
2 

1console.log('log-message'); // Outputs a normal information log to the console window
2console.warn('warn-message'); // Outputs warning in the console window
3console.error('error-message'); // Outputs error in the console window
4console.table('table-message'); // Outputs a table of all the object properties

1<script> 
2    console.log(x);       // return undefined
3    var x="hello js";       
4    console.log(x);        // return "hello js" 
5</script>    
6