Feb 15, 2017 7:41:35 AM | JavaScript Error Handling: SyntaxError: "use strict" not allowed in function with non-simple parameters

A detailed examination of the Invalid "Use Strict" with non-simple Parameters SyntaxError you might encounter in JavaScript.

Delving deeper into the misty (and mysterious) mountains of JavaScript Error Handling, we've come upon the frightening and dangerous lair of the beast known as the Invalid Strict with Complex Parameters error! [INSERT TERRIFIED SCREAMS]. The Invalid Strict with Complex Parameters error rears its ugly head anytime strict mode is enabled within a function that contains any sort of complex parameter.

Below we'll take a look at a few examples to show when Invalid Strict with Complex Parameters errors may appear, then take a deeper dive into dealing with such errors if they should pop up. Let's get to it!

The Technical Rundown

  • All JavaScript error objects are descendants of the Error object, or an inherited object therein.
  • The SyntaxError object is inherited from the Error object.
  • The Invalid Strict with Complex Parameters error is a specific type of SyntaxError object.

When Should You Use It?

Since the appearance of an Invalid Strict with Complex Parameters error in the first place indicates that we've enabled the use of strict mode, we should first take a few moments to examine what strict mode actually is. In short, strict mode is a toggled directive that forces JavaScript to behave in a slightly altered manner, usually by opting into less secure limitations placed upon the code, and thereby opening up execution to more dangers and exploits. While strict mode can lessen overall security, it can also be a requirement in certain coding situations, and in such cases, it's entirely possible to produce a Invalid Strict with Complex Parameters error.

The Invalid Strict with Complex Parameters error itself will appear when strict mode is enabled within a function that has one of the following parameter types:

  • Default Parameters
  • Rest Parameters
  • Destructuring Parameters

default parameter is when a parameter definition includes a default value, in the form of paramName = defaultValue. For example, here we're defining a default value of 99 for the age parameter in our addUser() function:

function addUser(name, age = 99) {
console.log(`Name is: ${name}, aged: ${age}`);


rest parameter is the JavaScript term for what's also known as a variadic function parameter -- a parameter that represents an indefinite number of arguments. In many languages, and JavaScript is no different, this variable number of parameters is represented by an ellipsis (...) prior to the variable name. For example, here our addUsers() function accepts an indeterminate number of parameters, which we defined as the argument names, each representing the name of a new user to add:

function addUsers(...names) {
names.forEach(function(element) {
console.log(`Name is: ${element}`);

addUsers('Alice', 'Bob', 'Chris', 'David');

We can then pass any number of names to the calling of our addUsers() function. As expected, these passed names will be automatically converted into an array of values, which are then output to the console as specified in our addUsers() function:

Name is: Alice
Name is: Bob
Name is: Chris
Name is: David

Lastly, the third type of parameter that can cause an Invalid Strict with Complex Parameters error is a destructuring parameter, or destructuring assignment. Destructuring is a syntax by which we can extract data from arrays or other objects into distinct variables. For example, rather than looping through each of our provided names from our last addUsers()example, maybe we just want to extract the first two names in the array, and assign them to unique variables:

function addUsers(...names) {
var [first, second] = names;
console.log(`First name is: ${first}`);
console.log(`Second name is: ${second}`);

addUsers('Alice', 'Bob', 'Chris', 'David');

By utilizing the [enclosing brackets] syntax to indicate our destructuring variables, we are expecting to extract the first and second values in the names array, and assign them to the first and second variables, respectively. Sure enough, the output matches:

First name is: Alice
Second name is: Bob

Now that we know what constitutes a type of complex parameter in JavaScript we can test it out by enabling strict mode and see how the engine reacts. Let's try it with our first example above:

var printError = function(error, explicit) {
console.log(`[${explicit ? 'EXPLICIT' : 'INEXPLICIT'}] ${error.name}: ${error.message}`);

try {
function addUser(name, age = 99) {
'use strict';
console.log(`Name is: ${name}, aged: ${age}`);

} catch (e) {
if (e instanceof SyntaxError) {
printError(e, true);
} else {
printError(e, false);

While we've surrounded the important code with a bit of extra fluff to make it easier to catch any errors, as expected, simply by adding the use strict declaration to our addUser() function, which contains a default parameter specification, we produce an Invalid Strict with Complex Parameters error:

Uncaught SyntaxError: Illegal 'use strict' directive in function with non-simple parameter list

To dive even deeper into understanding how your applications deal with JavaScript Errors, check out the revolutionary Airbrake JavaScript error tracking tool for real-time alerts and instantaneous insight into what went wrong with your JavaScript code.

Written By: Frances Banks