peek operation in stack using javascript

To check the topmost element unfortunately you must explicitly index it

var top = stack[stack.length-1];

the syntax stack[-1] (that would work in Python) doesn't work: negative indexes are valid only as parameters to slice call.

// The same as stack[stack.length-1], just slower and NOT idiomatic
var top = stack.slice(-1)[0];

To extract an element there is however pop:

// Add two top-most elements of the stack
var a = stack.pop();
var b = stack.pop();
stack.push(a + b);

var stack = [];
console.log(stack[stack.length-1]); // the top element
console.log(stack.length); //size