timers: Fail early when callback is not a function

`setTimeout()`, `setInterval()` and `setIntermediate` currently
throw errors when receiving non-function objects as their first
argument, but only do so when trying to execute the callback,
i.e. after the waited time has passed. This may complicate
debugging when a lot of calls to `setTimeout()`/etc. are involved,
so failing as early as possible seems like a good idea.

`setTimeout()` historically ignored an falsy first
argument, while the other functions do not and throw instead.
This patch changes this behaviour to make all three match and
adds remarks in the corresponding documentation.

Author: addaleax

doc/api/timers.markdown

@@ -37,6 +37,8 @@ The entire callback queue is processed every event loop iteration. If you queue
 an immediate from inside an executing callback, that immediate won't fire
 until the next event loop iteration.
 
+If `callback` is not a function `setImmediate()` will throw immediately.
+
 ## setInterval(callback, delay[, arg][, ...])
 
 To schedule the repeated execution of `callback` every `delay` milliseconds.
@@ -47,6 +49,8 @@ To follow browser behavior, when using delays larger than 2147483647
 milliseconds (approximately 25 days) or less than 1, Node.js will use 1 as the
 `delay`.
 
+If `callback` is not a function `setInterval()` will throw immediately.
+
 ## setTimeout(callback, delay[, arg][, ...])
 
 To schedule execution of a one-time `callback` after `delay` milliseconds. Returns a
@@ -62,6 +66,8 @@ To follow browser behavior, when using delays larger than 2147483647
 milliseconds (approximately 25 days) or less than 1, the timeout is executed
 immediately, as if the `delay` was set to 1.
 
+If `callback` is not a function `setTimeout()` will throw immediately.
+
 ## unref()
 
 The opaque value returned by [`setTimeout`][] and [`setInterval`][] also has the method

lib/timers.js

@@ -177,6 +177,10 @@ exports.enroll = function(item, msecs) {
 
 
 exports.setTimeout = function(callback, after) {
+  if (typeof callback !== 'function') {
+    throw new TypeError('"callback" argument must be a function');
+  }
+
   after *= 1; // coalesce to number or NaN
 
   if (!(after >= 1 && after <= TIMEOUT_MAX)) {
@@ -233,6 +237,10 @@ exports.clearTimeout = function(timer) {
 
 
 exports.setInterval = function(callback, repeat) {
+  if (typeof callback !== 'function') {
+    throw new TypeError('"callback" argument must be a function');
+  }
+
   repeat *= 1; // coalesce to number or NaN
 
   if (!(repeat >= 1 && repeat <= TIMEOUT_MAX)) {
@@ -419,6 +427,10 @@ Immediate.prototype._idlePrev = undefined;
 
 
 exports.setImmediate = function(callback, arg1, arg2, arg3) {
+  if (typeof callback !== 'function') {
+    throw new TypeError('"callback" argument must be a function');
+  }
+
   var i, args;
   var len = arguments.length;
   var immediate = new Immediate();

test/parallel/test-timers-throw-when-cb-not-function.js

@@ -0,0 +1,62 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+
+function doSetTimeout(callback, after) {
+  return function() {
+    setTimeout(callback, after);
+  };
+}
+
+assert.throws(doSetTimeout('foo'),
+  /"callback" argument must be a function/);
+assert.throws(doSetTimeout({foo: 'bar'}),
+  /"callback" argument must be a function/);
+assert.throws(doSetTimeout(),
+  /"callback" argument must be a function/);
+assert.throws(doSetTimeout(undefined, 0),
+  /"callback" argument must be a function/);
+assert.throws(doSetTimeout(null, 0),
+  /"callback" argument must be a function/);
+assert.throws(doSetTimeout(false, 0),
+  /"callback" argument must be a function/);
+
+
+function doSetInterval(callback, after) {
+  return function() {
+    setInterval(callback, after);
+  };
+}
+
+assert.throws(doSetInterval('foo'),
+  /"callback" argument must be a function/);
+assert.throws(doSetInterval({foo: 'bar'}),
+  /"callback" argument must be a function/);
+assert.throws(doSetInterval(),
+  /"callback" argument must be a function/);
+assert.throws(doSetInterval(undefined, 0),
+  /"callback" argument must be a function/);
+assert.throws(doSetInterval(null, 0),
+  /"callback" argument must be a function/);
+assert.throws(doSetInterval(false, 0),
+  /"callback" argument must be a function/);
+
+
+function doSetImmediate(callback, after) {
+  return function() {
+    setImmediate(callback, after);
+  };
+}
+
+assert.throws(doSetImmediate('foo'),
+  /"callback" argument must be a function/);
+assert.throws(doSetImmediate({foo: 'bar'}),
+  /"callback" argument must be a function/);
+assert.throws(doSetImmediate(),
+  /"callback" argument must be a function/);
+assert.throws(doSetImmediate(undefined, 0),
+  /"callback" argument must be a function/);
+assert.throws(doSetImmediate(null, 0),
+  /"callback" argument must be a function/);
+assert.throws(doSetImmediate(false, 0),
+  /"callback" argument must be a function/);