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# Considerations for switching JavaScript modes

# Considerations for switching JavaScript modes {#ariaid-title1}

* Release version: Xanadu
* 
* Updated August 1, 2024
* 
* ![](https://www.servicenow.com/docs/portal-asset/ico-clock) 3 minutes to read

Switching the JavaScript mode for an application or script might change the behavior of existing scripts. Review some examples of behavior changes before switching JavaScript modes or to troubleshoot
any issues that you experience after switching.{#considerations-switching-javascript-mode__otherprops-integration-4}

For more information about each JavaScript mode, see [JavaScript modes](https://servicenow-prod.fluidtopics.net/4MIkVSVOsgrYS~qMY6BfXA "JavaScript mode is a design and runtime setting for custom applications and scripts. To support existing server-side scripts and new scripts developed to the ECMAScript 2021 standard, the JavaScript engine has three modes: ECMAScript 2021 (ES12), ES5 Standards, and Compatibility.") and [JavaScript engine feature support](https://servicenow-prod.fluidtopics.net/nJGoD1YjFWtw8YbgWTYeXQ "Compare ECMAScript features between the ECMAScript 2021 (ES12) and ES5 Standards JavaScript modes in Xanadu. Both modes support a subset of ECMAScript features.").

This table highlights how JavaScript behavior has evolved from the lenient and error-prone
pre-ES5 environment, to the stricter and more predictable ES5, and lastly the more feature-rich
environment of ES12 (ECMAScript 2021).  
{#considerations-switching-javascript-mode__table_r4b_j4r_fcc__entry__4}

| Feature | Compatibility Mode | ES5 Standards Mode | ECMAScript 2021 (ES12) |
|-|-|-|-|
| Arguments object | The `arguments` object exists, but there's no `strict mode`, so modifications reflect on `arguments`. Prints: *** Script: [object Arguments] *** Script: [object Arguments] *** Script: [object Arguments] *** Script: 123 | In strict mode, the arguments object doesn't reflect parameter modifications and throws an error. Prints: sn_es5: 123 sn_es5: undefined sn_es5: [object Arguments] sn_es5: 123 | The same as ES5. |
| Boolean overrides | Primitive Booleans (`true`, `false`) can be overridden, causing unexpected behavior. | Primitive Booleans are more protected, though still can be overridden when assigned to variables. | The same as ES5, but strict mode helps prevent some assignments. The conditional expression should be written in this form: (cond_expr instanceof Boolean ? cond_expr.valueOf() : cond_expr). |
| Exception for syntax errors | Syntax errors throw exceptions at runtime. Error handling is inconsistent. Example: Javascript compiler exception: unterminated string literal (null.null.script; line 1) in: var b = ' | More consistent syntax error handling, especially in strict mode. Example: Evaluator: com.glide.script.RhinoEcmaError: unterminated string literal script : Line(1) column(9) ==>   1: var b = ' | The same as ES5, but with more robust handling and clearer error messages in updated engines. Example: SyntaxError: Unterminated string constant at line 1 ==>   1: var b = ' |
| Increment and decrement | Allowed on variables but could behave unexpectedly with complex expressions. Prints: *** Script: c: 1 *** Script: gr.related_incidents: 1 *** Script: 2 *** Script: 3 | Improved clarity, but still allowed on variables (`var`, `let`, `const`). Prints: sn_es5: c: 0 sn_es5: gr.related_incidents: 1 sn_es5: 1 sn_es5: 2 | The same as ES5, with stricter rules in some contexts (for example, `const`). |
| Line continuations | Allowed with a backslash (`\`) but discouraged due to readability issues. In this example, all three functions are called. var expr = doFoo();  // do foo doBar();  // do bar finish();   // all done eval(expr); | Same as Compatibility mode; no change in handling line continuations. In the previous example, ES5 only calls the first function and treats everything after the first comment including the newline as comment until the expression end. | The same as ES5, but template literals provide a more readable alternative. |
| Missing semicolons | Automatic semicolon insertion (ASI) often led to unexpected behavior. | Throws a syntax error when a semicolon is missing. | The same as ES5. Updated practices encourage explicit semicolons. |
| Non-existent functions | Calling a non-existent function throws a `ReferenceError`. | Throws a `TypeError` if a non-function is called. | Throws an EcmaError when a non-existent function is called or a property is referenced. |
| Non-existent properties | Accessing a non-existent property returns `undefined`; no error thrown. | Same as pre-ES5. | The same as Compatibility mode and ES5 Standards mode. |
| Numeric literals | Basic decimal and hexadecimal literals. | Introduced stricter parsing rules and better handling of numeric literals. | Added binary (`0b`), octal (`0o`), and BigInt literals (`123n`). |
| Reserved keyword as property | Using reserved keywords isn't possible. | Reserved keywords can be used as property names without error, for example, `obj.for`. Prints the object when returned. | The same as ES5. |
| Treat let and yield as keywords | `let` and `yield` aren't keywords and can be used as identifiers only. | `let` is introduced as a keyword. `yield` is reserved in strict mode. | Both are keywords. Using them as identifiers throws syntax errors. |
[Table 1. Behavioral differences in JavaScript modes]

{#considerations-switching-javascript-mode__table_r4b_j4r_fcc}