I bet this one will surprise you — but this one is easier than you think. Thru is just an informal spelling of the preposition, adverb, and adjective definitions of the word Through. Here are all the definitions of both spellings:
preposition & adverb
1 moving in one side and out of the other side of (an opening, channel, or location) : [as prep. ] stepping boldly through the doorway | [as adv. ] as soon as we opened the gate, they came streaming through.
• so as to make a hole or opening in (a physical object) : [as prep. ] the truck smashed through a brick wall | [as adv. ] a cucumber, slit, but not all the way through.
• moving around or from one side to the other within (a crowd or group) : [as prep. ] making my way through the guests.
• so as to be perceived from the other side of (an intervening obstacle) : [as prep. ] the sun was streaming in through the window | [as adv. ] the glass in the front door where the moonlight streamed through.
• [ prep. ] expressing the position or location of something beyond or at the far end of (an opening or an obstacle) : the approach to the church is through a gate.
• expressing the extent of turning from one orientation to another : [as prep. ] each joint can move through an angle within fixed limits.
2 continuing in time toward completion of (a process or period) : [as prep. ] he showed up halfway through the second act | [as adv. ] to struggle through until payday.
• so as to complete (a particular stage or trial) successfully : [as prep. ] she had come through her sternest test | [as adv. ] I will struggle through alone rather than ask for help.
• from beginning to end of (an experience or activity, typically a tedious or stressful one) : [as prep. ] we sat through some very boring speeches | she’s been through a bad time | [as adv. ] Karl will see you through, Ingrid.
3 so as to inspect all or part of (a collection, inventory, or publication) : [as prep. ] flipping through the pages of a notebook | [as adv. ] she read the letter through carefully.
4 [ prep. ] up to and including (a particular point in an ordered sequence) : they will be in town from March 24 through May 7.
5 [ prep. ] by means of (a process or intermediate stage) : dioxins get into mothers’ milk through contaminated food.
• by means of (an intermediary or agent) : seeking justice through the proper channels.
6 [ adv. ] so as to be connected by telephone : he put a call through to the senator.
1 [ attrib. ] (of a means of public transportation or a ticket) continuing or valid to the final destination : a through train from Boston.
2 [ attrib. ] denoting traffic that passes from one side of a place to another in the course of a longer journey : neighborhoods from which through traffic would be excluded.
• denoting a road that is open at both ends, allowing traffic free passage from one end to the other : the shopping center is on a busy through road.
3 [ attrib. ] (of a room) running the whole length of a building.
4 [ predic. ] informal having no prospect of any future relationship, dealings, or success : she told him she was through with him | you and I are through.
through and through in every aspect; thoroughly or completely : Harriet was a political animal through and through.
preposition, adverb, & adjective
informal spelling of through
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*Source: Apple Dictionary