The JavaScript strict mode-only exception
"for-in loop head declarations may not have initializers"
occurs when the head of a for...in contains
an initializer expression, such as for (var i = 0 in obj). This is not
allowed in for-in loops in strict mode. In addition, lexical declarations with initializers like for (const i = 0 in obj) are not allowed outside strict mode either....The JavaScript strict mode-only exception
"for-in loop head declarations may not have initializers"
occurs when the head of a for...in contains
an initializer expression, such as for (var i = 0 in obj). This is not
allowed in for-in loops in strict mode. In addition, lexical declarations with initializers like for (const i = 0 in obj) are not allowed outside strict mode either.