TypeScript Language Specification v1.5

In the example
var x = 1;
function f(a = x) {
var x = "hello";
}
the local variable 'x' is in scope in the parameter initializer (thus hiding the outer 'x'), but it is an error to
reference it because it will always be uninitialized at the time the parameter initializer is evaluated.
6.4 Destructuring Parameter Declarations
Parameter declarations can specify binding patterns (section 3.8.2.2) and are then called destructuring
parameter declarations. Similar to a destructuring variable declaration (section 5.1.2), a destructuring
parameter declaration introduces zero or more named locals and initializes them with values extracted
from properties or elements of the object or array passed as an argument for the parameter.
The type of local introduced in a destructuring parameter declaration is determined in the same manner
as a local introduced by a destructuring variable declaration, except the type T associated with a
destructuring parameter declaration is determined as follows:
If the declaration includes a type annotation, T is that type.
Otherwise, if the declaration includes an initializer expression, T is the widened form (section 3.11)
of the type of the initializer expression.
Otherwise, if the declaration specifies a binding pattern, T is the implied type of that binding
pattern (section 5.1.3).
Otherwise, if the parameter is a rest parameter, T is any[].
Otherwise, T is any.
When the output target is ECMAScript 6 or higher, except for removing the optional type annotation,
destructuring parameter declarations remain unchanged in the emitted JavaScript code. When the output
target is ECMAScript 3 or 5, destructuring parameter declarations are rewritten to local variable
declarations.
The example
function drawText({ text = "", location: [x, y] = [0, 0], bold = false }) {
// Draw text
}
declares a function drawText that takes a single parameter of the type
{ text?: string; location?: [number, number]; bold?: boolean; }
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