Prevent Node.js repl from printing output

Why don't you just append ; null; to your expression?

As in

new Array(10000); null;

which prints

null

or even shorter, use ;0;

Assign the result to a variable declared with var. var statements always return undefined.

> new Array(10)
[ , , , , , , , , ,  ]

> var a = new Array(10)
undefined

I have already said in a comment to this question that you may want to wrap the execution of your command in an anonymous function. Let's say you have some repeated procedure that returns some kind of result. Like this:

var some_array = [1, 2, 3];

some_array.map(function(){

    // It doesn't matter what you return here, even if it's undefined
    // it will still get into the map and will get printed in the resulting map
    return arguments;
});

That gives us this output:

[ { '0': 1,
    '1': 0,
    '2': [ 1, 2, 3 ] },
  { '0': 2,
    '1': 1,
    '2': [ 1, 2, 3 ] },
  { '0': 3,
    '1': 2,
    '2': [ 1, 2, 3 ] } ]

But if you wrap the map method call into a self-invoking anonymous function, all output gets lost:

(function(){
    some_array.map(function() {
        return arguments;
    });
})();

This code will get us this output:

undefined

because the anonymous function doesn't return anything.

You could start the REPL yourself and change anything that annoys you. For example you could tell it not to print undefined when an expression has no result. Or you could wrap the evaluation of the expressions and suppress the return values. If you do both of these things at the same time you effectively reduce the REPL to a REL:

node -e "var vm = require('vm'); require('repl').start({ignoreUndefined: true, eval: function(cmd, ctx, fn, cb) {var err = null; try {vm.runInContext(cmd, ctx, fn);} catch (e) {err = e;} cb(err);}})"

Node uses inspect to format the return values. Replace inspect with a function that just returns an empty string and it won't display anything.

require('util').inspect = function () { return '' };