TypeScript Language Specification v1.5

In a destructuring assignment expression, the type of the expression on the right must be assignable to
the assignment target on the left. An expression of type S is considered assignable to an assignment
target V if one of the following is true:
V is variable and S is assignable to the type of V.
V is an object assignment pattern and, for each assignment property P in V,
o S is the type Any, or
o S has an apparent property with the property name specified in P of a type that is
assignable to the target given in P, or
o P specifies a numeric property name and S has a numeric index signature of a type that is
assignable to the target given in P, or
o S has a string index signature of a type that is assignable to the target given in P.
V is an array assignment pattern, S is the type Any or an array-like type (section 3.3.2), and, for
each assignment element E in V,
o S is the type Any, or
o S is a tuple-like type (section 3.3.3) with a property named N of a type that is assignable
to the target given in E, where N is the numeric index of E in the array assignment
pattern, or
o S is not a tuple-like type and the numeric index signature type of S is assignable to the
target given in E.
In an assignment property or element that includes a default value, the type of the default value must be
assignable to the target given in the assignment property or element.
When the output target is ECMAScript 6 or higher, destructuring variable assignments remain unchanged
in the emitted JavaScript code.
When the output target is ECMAScript 3 or 5, destructuring variable assignments are rewritten to series of
simple assignments. For example, the destructuring assignment
var x = 1;
var y = 2;
[x, y] = [y, x];
is rewritten to the simple variable assignments
var x = 1;
var y = 2;
_a = [y, x], x = _a[0], y = _a[1];
var _a;
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