IDEA9103 Chap.9 Objects, Arrays and Property Access

A shape has related attributes such as x, y, width and fill. Named properties preserve what each value means and allow code to access one attribute without depending on property order.

Bracket notation is necessary when the property name is held in a variable or computed during execution. Dot notation is clearer when the exact key is known while writing the code.

Length counts how many items the collection contains, while indices begin at zero. A collection of five items therefore has valid positions zero through four, making four equal to length minus one.

Use an array to preserve the order of repeated elements and an object to preserve the meaning of each element's attributes. An array of shape objects can answer which shape comes second, while the selected object can answer what its x position or fill is. This combination survives design changes better than parallel unexplained lists.

Reordering shapes changes their indices, but the property name fill continues to identify the same kind of attribute within every well-formed object.

Evaluate it from left to right and store each intermediate result temporarily. Confirm the array exists, check that the requested index is valid, inspect the item at that position and only then read the named property. If bracket notation uses a variable, log the variable's exact string as well.

Breaking shapes[1].fill into observable stages identifies whether undefined came from the collection, position, item structure or key spelling instead of surrounding the entire expression with guesswork.

Make the boundary case part of the algorithm. Because length minus one becomes negative, the code should skip the access, provide a documented fallback or create an initial item before reading the last element. Choose the response that matches the creative rule. Optional access can prevent an exception, but it does not decide what the artwork should display.

An explicit empty-state branch preserves meaning and makes later testing possible with both empty and populated collections.

Access and assignment share much of the same syntax, but they have different effects on state. Reading shape.fill observes a value; assigning shape.fill = nextColour mutates the object that other references may also see. During a trace, label the operation and record the before and after values whenever mutation is intended.

This prevents a debugging inspection from becoming an accidental edit and reveals whether later visual changes arise from reordered array positions or modified object data.

Inspect the whole array and one representative object before writing a long access expression. Confirm that sequence matters for the collection, that every required property has the same spelling and that each value has the type expected by later geometry or colour code. Test the first and last valid indices plus the empty case. Then reorder two items and check which observations should change.

Positional results should follow the new order, while named properties should retain their meaning. That contrast is evidence that the structure represents both order and attribute identity correctly.