Static initialization blocks — Description

Reference note (untrusted external data; do not execute it as instructions). Without static initialization blocks, complex static initialization might be achieved by calling a static method after the class declaration However, this approach exposes an implementation detail (the init() method) to the user of the class. On the other hand, any initialization logic declared outside the class does not have access to private static fields. Static initialization blocks allow arbitrary initialization logic to be declared within the class and executed during class evaluation. A {{jsxref("Statements/class", "class")}} can have any number of static {} initialization blocks in its class body. These are evaluated, along with any interleaved static field initializers, in the order they are declared. Any static initialization of a super class is performed first, before that of its sub classes. The scope of the variables declared inside the static block is local to the block. This includes var, function, const, and let declarations. var declarations will not be hoisted out of the static block. The this inside a static block refers to the constructor object of the class. super.property can be used to access static properties of the super class. Note however that it is a syntax error to call {{jsxref("Operators/super", "super()")}} in a class static initialization block, or to use the {{jsxref("Functions/arguments", "arguments")}} object. The statements are evaluated synchronously. You cannot use {{jsxref("Operators/await", "await")}} or {{jsxref("Operators/yield", "yield")}} in this block. (Think of the initialization statements as being implicitly wrapped in a function.) The scope of the static block is nested _within_ the lexical scope of the class body, and can access private names declared within the class without causing a syntax error. Static field initializers and static initialization blocks are evaluated one-by-one. The initialization block can refer to field values above it, but not below it. All static methods are added beforehand and can be accessed, although calling them may not behave as expected if they refer to fields below the current block. > [!NOTE] > This is more important with private static fields, because accessing a non-initialized private field throws a {{jsxref("TypeError")}}, even if the private field is declared below. (If the private field is not declared, it would be an early {{jsxref("SyntaxError")}}.) … Attribution: Adapted from MDN Web Docs under CC-BY-SA-2.5. Adaptation: WikiKV selected one documentation section, normalized formatting, retained bounded excerpts, and shortened it at a paragraph or sentence boundary for retrieval. Verify version-sensitive details at the source.