JavaScript modes

  • Release version: Yokohama
  • Updated January 30, 2025
  • 2 minutes to read
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    Summary of JavaScript modes

    JavaScript mode in ServiceNow defines the design and runtime behavior for custom applications and scripts by controlling which JavaScript features are available. This enables compatibility and support for different ECMAScript standards across applications. There are three modes: ECMAScript 2021 (ES12), ES5 Standards, and Compatibility. Each mode targets specific ECMAScript versions and legacy support requirements, affecting script capabilities and behavior.

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    JavaScript Modes Explained

    • ECMAScript 2021 (ES12) mode: This is the default for new scoped applications and new scripts, regardless of the application’s mode. It supports modern ECMAScript 2021 and some ECMAScript 2022 features, including default function parameters, rest parameters, for-of loops, template literals, destructuring, arrow functions, classes, Map and Set objects, and optional chaining (?.). This mode does not preserve legacy behaviors from before the Tokyo release and is incompatible with global scripts.
    • ES5 Standards mode: The default for global applications and an option for scoped applications, it supports ECMAScript 5 features like strict mode ("use strict"), object property control, accessors, new Array and Date methods, native JSON support, and compatibility with modern third-party libraries such as lodash.js and moment.js. It does not preserve legacy pre-Helsinki engine behaviors.
    • Compatibility mode: Used by scripts developed before ES5 Standards mode was introduced. It differs from the legacy engine mainly in JSON support, where JSON.stringify() and JSON.parse() use the native ES5 JSON object. Legacy JSON().encode() and JSON().decode() methods remain but should be used only for legacy behavior. Third-party JavaScript libraries are not supported in this mode.

    Practical Considerations for ServiceNow Customers

    • Switching modes: Changing the JavaScript mode for an application or individual script can alter script behavior and may require testing and troubleshooting. Customers should review potential behavior changes before switching modes.
    • Configuring application mode: You can configure which ECMAScript features are available by selecting the JavaScript mode at the application level.
    • Enabling ES12 for individual scripts: Even in applications using ES5 Standards or Compatibility modes, you can enable ECMAScript 2021 (ES12) mode for individual server-side scripts to leverage the latest features where needed.

    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.

    The JavaScript mode controls which JavaScript features you have access to in an application or script. The default mode for new scoped applications is ECMAScript 2021 (ES12) and for new global applications, it’s ES5 Standards. You can also turn on ECMAScript 2021 (ES12) mode for individual scripts in applications that use ES5 Standards or Compatibility mode.

    For more information about features supported by the ECMAScript 2021 (ES12) and ES5 Standards modes, see JavaScript engine feature support.

    ECMAScript 2021 (ES12) mode

    ECMAScript 2021 (ES12) mode is the default mode when you create new scoped applications. When you create new scripts, ECMAScript 2021 (ES12) mode is turned on by default regardless of the JavaScript mode configured for the application. This mode doesn’t preserve the legacy behaviors in the pre-Tokyo JavaScript engine or work with global scripts.

    ECMAScript 2021 (ES12) mode supports a subset of ECMAScript 2021 (ES12) and ECMAScript 2022 (ES13) syntax and features, including the following features:
    • Default function parameters
    • Rest parameters
    • For-of loops
    • Template literals
    • Destructuring
      • Declarations
      • Assignment
      • Parameters
    • Const declaration
    • Let declaration
    • Arrow functions
    • Class declarations
    • Map set
    • Optional chaining operator (?.)

    To learn about specific ECMAScript 2021 (ES12) features, see the Let's Learn ECMAScript 2021 videos on the ServiceNow Dev Program YouTube channel.

    ES5 Standards mode

    ES5 Standards mode is the default mode for global applications and is an option for scoped applications. This mode doesn’t preserve the legacy behaviors in the pre-Helsinki JavaScript engine.

    ES5 standards mode supports ECMAScript5 syntax and features, including the following features:
    • The "use strict" declaration
    • Control over extensibility of objects
    • Get and set properties on objects (accessors)
    • Control over writability, configurability, and enumerability of object properties
    • New Array and Date methods
    • Native JSON support
    • Support for modern third-party libraries such as lodash.js and moment.js

    Compatibility mode

    Compatibility mode is used for all scripts developed prior to the addition of ES5 Standards mode. Compatibility mode has some differences from the previous JavaScript engine.

    JSON support changes:
    • JSON.stringify() and JSON.parse() are implemented using the ES5 Native JSON object.
    • The new JSON().encode() and new JSON().decode() are still supported, but should only be used when the legacy behavior is required.

    The use of third-party JavaScript libraries isn’t supported in Compatibility mode.