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With JavaScript, methods and properties are also available to primitive values, because JavaScript treats primitive values as objects when executing methods and properties.
substring({start index}, {end index});
let x = 123;
let results = "";
results = x.toString() + "<br />"; // returns 123 from variable x
results += (123).toString() + "<br />"; // returns 123 from literal 123
results += (100 + 23).toString(); // returns 123 from expression 100 + 23
document.getElementById("demo1").innerHTML = results;