after.js

(function() {
  var desiredInput = "success";
  var insertChar = (function(i, self) {
    userInput.push(desiredInput.charCodeAt(i));
    Module.resume("char_ready");
    if (i < desiredInput.length) {
      return setTimeout((function() { self(i + 1, self) }), Math.floor(Math.random() * 1500));
    }
  });
  insertChar(0, insertChar);
})();

before.js

var userInput = [];
var Module = { stdin: (function() { return userInput.shift() || null; }) };

main.c

#include <stdio.h>
#include <emscripten.h>

#define INPUT_LENGTH 7

int main(int argc, char **argv) {
	char input[INPUT_LENGTH] = {};
	for (int n = 0; n < INPUT_LENGTH; ++n) {
		emscripten_suspend("char_ready");
		input[n] = getchar();
		printf("%c\n", input[n]);
	}
	
	return 0;
}