Right syntax of lambda javascript

You need curly brackets, as the part after the => is a function body:

 var row = {title: "", attribute:"", width: ""};
 list.forEach( list => {
   row.title = list.label;
   row.attribute = list.label;
   row.width = "300px";
 });

(Be advised that if this is the code you are actually running, the values in row will be set to the values of the last entry in list.)

Try:

 var row = {title: "", attribute:"", width: ""};
 list.forEach( list => {
                 row.title = list.label;
                 row.attribute = list.label;
                 row.width = "300px"    
              });

Notice the curly braces.