一个字面量就是在源代码中表示某个固定值的符号.
javascript Literal(字面量、直接量),标准文档包含:
ArrayLiteral
BooleanLiteral
DecimalIntegerLiteral
DecimalLiteral
HexIntegerLiteral
JSONBooleanLiteral
JSONNullLiteral
NullLiteral
NumericLiteral
ObjectLiteral
OctalIntegerLiteral
RegularExpressionLiteral
StrDecimalLiteral
StringLiteral
StringNumericLiteral
StrNumericLiteral
StrUnsignedDecimalLiteral
注意:undefined不是literal.
tips: window.undefined === undefined 只是一个全局属性,非保留字。类似的还有NaN、Infinity.