What is a Lambda in Javascript ?

Lambda can have different meanings depending on who you're talking to or what you're talking about. In the context of JavaScript it usually refers to an anonymous function. That is a function that doesn't get named, but is usually used as a value passed to another function in order pass a behavior as a value.

Here's an example. The Array.prototype.sort() function sorts an array. You can pass a function to define how to sort. In this example we are sorting by the name property of the object. In this example the lambda is a function that will receive 2 parameters (the 2 items to compare for the sort) and it should return a numeric value (negative for a before b, 0 for equal, positive for a after b)

var names = [
  {name: "Jim Hoskins", id: 1},
  {name: "Guil Hernandez", id: 2},
  {name: "Ben Jakuben", id:3}
];

names.sort(function (a, b) {
  if (a.name < b.name) {
    return  -1;
  } else if (a.name > b.name) {
    return 1;
  } else {
    return 0;
  }
});

Generally we refer to them as "anonymous functions" in JavaScript, but in this case lambda would be a synonym.