When Parsing {}, Interprets It As What: Block or Object?

Block vs. Object in JavaScript's Interpretation of {}

When parsing code, JavaScript faces the ambiguity of interpreting "{}" either as an empty block or an empty object. To determine which interpretation to use, JavaScript follows the grammar defined in its specifications.

According to the language specification, a Statement can take the form of a Block, a VariableStatement, an EmptyStatement, or an ExpressionStatement. A Block is defined as "{}" followed by an optional list of Statements terminated by "}".

Therefore, when JavaScript encounters "{}", it first considers it to be a Block because "{}" matches the syntax for a Block. Only if the parser cannot interpret it as a Block will it then consider the {} as an empty object, which falls under the category of ExpressionStatement.

In the example provided, "{}[]" is interpreted as an empty block followed by a unary plus and an empty array. The empty block does nothing, the array is converted to an empty string, which is then converted to a number (0).

Firebug interprets this input as a Statement, resulting in an empty block that evaluates to undefined. On the other hand, Node.js treats it as an expression, unable to parse it as a Block, and evaluates to {}.

This discrepancy arises from different interpretations of the input by the respective JavaScript engines. Firebug and Chrome dev tools treat it as a Statement, considering the {} as an empty block, while Node.js treats it as an expression, resulting in an empty object.

The above is the detailed content of When Parsing {}, Interprets It As What: Block or Object?. For more information, please follow other related articles on the PHP Chinese website!